GB2421375A

GB2421375A - A ramp generator for an analogue to digital converter

Info

Publication number: GB2421375A
Application number: GB0427819A
Authority: GB
Inventors: Yan Lee
Original assignee: Micron Europe Ltd
Current assignee: Micron Europe Ltd
Priority date: 2004-12-17
Filing date: 2004-12-17
Publication date: 2006-06-21
Anticipated expiration: 2024-12-17
Also published as: GB2421375B; GB0427819D0

Abstract

A ramp generator for an analog-to-digital converter comprises an array of capacitors each controlled by a switch responsive to one or more control signals and operable to connect/disconnect one or more of the capacitors relative to the array and a current source operable to charge at least one of the capacitors. Operating the ramp generator comprises resetting the ramp generator, enabling a current generator to charge at least one capacitor in the switched capacitor array, and controlling the state of one or more switches, wherein the switches are operable to connect and disconnect one or more of the capacitors relative to the array. The output of the ramp generator having a plurality of programmable breakpoints. The analogue to digital converter may also comprise means to produce a falling ramp signal and a rising ramp signal.

Description

PROGRAMMABLE INTEGRATING RAMP GENERATOR AND METHOD OF OPERATING THE SAME BACKGROUND The present invention relates generally to a ramp generator for analog-to-digital (A/D) conversion applications and more particularly to a programmable nonlinear ramp generator which utilizes a switched-capacitor array for A/D conversion applications. An analog-to-digital converter (ADC) may be used to translate an analog signal (e.g., a current or voltage produced by a sensor) into a digital signal that can be used by another device (for example, a microprocessor). In complimentary metal-oxide semiconductor (CMOS) imaging applications, for example, ADCs are increasingly being used as the preferred means for converting charge captured by CMOS sensors into a digital read-out. Several types of ADCs are currently used, each type differing in the technique utilized to complete the A/D conversion. For example, feed-back type converters, dual-slope converters, flash converters, charge-redistribution converters, and digital ramp converters are known in the art. A feedback-type converter typically employs a comparator, an up-down counter, and a digital-to-analog converter (DAC). FIG. 8 is a simplified diagram of a prior art feedback-type converter. An analog signal (VA) is fed to one input of the comparator. The output of the comparator is connected to the input of the counter. The outputs of the counter are connected to the inputs of the DAC and the output of the DAC (V0) is fed back to another of the comparator's inputs. The counter also receives a clock signal. Whenever the output of the comparator is high (i.e., when the difference between VAand V0is positive), the counter counts the pulses of the clock signal and the output of the counter increases. This in turn causes the voltage V0to rise. When V0equals VA, the output of the comparator goes low and the counter is stopped. At this point, the counter's output represents the digital equivalent of the analog signal voltage. FIG. 9 is a simplified diagram of a prior art dual-slope converter. A dual-slope converter typically functions in two stages. In the first stage, an analog signal (VA) is applied for a fixed time period to charge the capacitor Ci and produce a voltage vi. The voltage Vi typically has a variable slope during this first stage. In the second stage, a reference signal (Vref) is applied for a variable time period and allows the voltage v(to discharge from the capacitor Ci. The voltage Vi typically has a constant slope during the second stage. Control logic provides signals to control switching between the first and second stages. The control logic also provides control signals to a counter which is used to count pulses from a fixed-frequency clock. The count recorded by the counter during the second stage represents the digital equivalent of the analog voltage applied during stage 1. FIG. 10 is a simplified diagram of a prior art flash converter. A flash converter typically uses 2<N">' comparators to simultaneously compare the analog input signal level (VA) to each of the 2<N">' possible quantization levels. A 4-bit DAC, for example, uses sixteen comparators to convert an analog signal into a 4-bit digital word. The DAC includes a logic block that encodes the output from each of the sixteen comparators into the N-bits of the digital word. For instance, an analog input signal between 0 and 5V may be represented using the 4-bit binary number. The 4-bit binary number may represent 2<4>(i.e., 16) different values (i.e., from 0 to 15). The resolution of the conversion will thus be 5V/15 = 1/3 V. Accordingly, the first quantization level (e.g., for bit 0000) corresponds to an analog signal of 0V, the second quantization level (e.g., for bit 0001) corresponds to an analog signal of 1/3V, the third quantization level (e.g., for bit 0010) corresponds to an analog signal of 2/3 V, and so on. This pattern is repeated for each of the sixteen quantization levels (i.e., up to bit 1111, which corresponds to an analog signal of 5 V). FIG. 11 is a simplified diagram of a prior art charge-redistribution converter. A charge-redistribution converter typically uses a capacitor array, a comparator, switches, and control logic, among others. During operation, a voltage (vA) proportional to the analog input voltage (VA) is first stored across the capacitors in the capacitor array by connecting one side of the array to VAand the other side of the array (e.g., the side also connected to an input of the comparator) to ground. The plates of capacitors connected to the input terminal of the comparator are then open-circuited (e.g., switch S2 is opened) while the plates of the capacitors on the other side of the capacitor array are switched to ground (e.g., SCI, SC2, ... SC6 are connected to ground). Next, the charge stored by the capacitors is redistributed by switching the individual capacitors to the reference voltage and/or ground until the voltage across the plates of the capacitors reaches zero. The final position of the switches (i.e., SCI, SC2, ... SC6) represents the output of the digital word. For example, a switch that is connected to ground in its final position represents a "0"; whereas a switch connected to the reference voltage source in its final position represents a "1". A digital ramp converter typically includes a comparator and a ramp generator. An analog signal is fed to one input of the comparator and the output of the ramp generator is fed to another input of the comparator. FIGS. 12 is a simplified circuit diagram of a ramp generator 100 and a comparator 102 according to the prior art. FIG. 13 is a timing diagram for a ramp generator 100 of FIG. 12 according to the prior art. The ramp generator 100 is comprised of a plurality of identical switching current sources 101(1), 101(2), 101(3), ... 101(n), a capacitor 103, and reset switch So- Operation begins by placing the ramp generator 100 into the reset mode by opening switches Si, S2, S3, ... Snand closing switch So- Referring to FIG. 13, at to, signal T0goes high and signals T) through Tnremain low (which keep switches 101(1) through 101 (n) open). When signal T0goes high, switch S0is closed and the output of the ramp generator 100 is connected to the voltage source Vref(i.e., Vrampequals Vref). At ti, signal T0goes low opening switch S0and signal Ti goes high closing switch Si and enabling current source 101(1). Current source 101(1) charges capacitor 103 and the output of the ramp generator (i.e., Vramp) begins to rise above Vrer at a constant rate which is proportional to the value of the current source 101(1). At the first break point, t2, signal T2goes high closing switch S2and enabling current source 101(2). The slew rate of the ramp generator output is now doubled. At the next break point, t3, signal T3goes high closing switch S3and enabling current source 101(3). This increases the slew rate of the ramp generator again. The final break point occurs at tnwhen the last current source 101(n) is enabled by signal Tnclosing switch Sn. Returning to FIG. 12, the output of the ramp generator (i.e., Vramp) is supplied to an input terminal comparator 102. Comparator 102 compares V ramp with an analog signal Va, which is supplied to another input terminal of the is ramp comparator 102. If Vais greater than Vramp, the output of comparator 102 is high and the ramp generator 100 continues to increase Vramp. If Vrampis greater than Va, the output of the comparator 102 goes low and the ramp generator 100 stops increasing Vramp. An ADC code counter (not shown) is used to stop the ramp and to determine the ADC code. One major drawback inherent to prior art ramp generators 100, however, is the difficulty encountered in trying to manufacture matched current generators. Due to the manufacturing techniques used to construct the transistors comprising the current sources, a current source can typically only be matched within approximately 2% of another current source. The inability to accurately match current source leads to inaccurate conversion of the analog signal. As discussed above, ADCs are increasingly being used as the preferred means for converting charge captured by CMOS sensors into a digital read-out in CMOS imaging applications. The error inherent in the prior art ADCs adversely effects the results obtained in the CMOS imaging applications. Accordingly, a need exists for a modulating ramp A/D converter which overcomes these problems and which overcomes other limitations inherent in the prior art. More specifically, a need exists for a modulating ramp A/D converter which can be used in CMOS imaging applications, for example, to convert charge captured by CMOS sensors into a digital read-out. SUMMARY According to a first aspect of the present invention, there is provided a ramp generator for an analog-to-digital converter, comprising: an array of capacitors each controlled by a switch operable to connect/disconnect one or more of said capacitors within said array, wherein each of said switches is responsive to one or more control signals; and a current source operable to charge at least one of said plurality of capacitors within said array. Preferably, the ramp generator comprises a re-set switch and a nonswitchable capacitance. Conveniently, the ramp generator further comprises a current source switch operable to disable said current source relative to said array. Advantageously, said capacitors are matched. Preferably, the ramp generator further comprises another current source for providing a bias function; and another current source switch operable to disable said another current source relative to said array. Advantageously, the ramp generator further comprises an operational amplifier, said operational amplifier having: a first input for receiving a reference voltage; a second input connected to at least one of an output of said current source, an output of said another current source, and a first side of said array; and an output connected to a second side of said array. Conveniently, said current source and said another current source are supplied by a regulated voltage. Preferably, the ramp generator is operable to produce an output having a linear portion and a compressed portion, wherein said compressed portion has a plurality of discreet segments, each segment being defined by one or more programmable breakpoints. Advantageously, the ramp generator comprises a falling ramp portion; and a rising ramp portion. Conveniently, said falling ramp portion comprises a first operational amplifier and said rising ramp portion comprises a second operational amplifier, said first operational amplifier having: an input for receiving a first reference voltage; and an input connected to at least one of a first current source, a first bias current source, and a first side of a first capacitance; and an output connected to a second side of said first capacitance; said second operational amplifier having: an input for receiving a second reference voltage; an input connected to at least one of a second current source, a second bias current source, and a first side of a second capacitance; and an output connected to a second side of said second capacitance. Preferably, said first capacitance comprises at least one of a first variable capacitance or a first array of capacitors wherein each capacitor is controlled by a switch and wherein said second capacitance comprises at least one of a second variable capacitance or a second array of capacitors wherein each capacitor is controlled by a switch. Advantageously, each of said switches for said first array is operable to connect/disconnect one or more of a said capacitors within said first array, each of said switches responsive to one or more control signals. Conveniently, each of said switches for said second array is operable to connect/disconnect one or more of a said capacitors within said second array, each of said switches responsive to one or more control signals. Preferably, at least one of said first current source, said first bias current source, said second current source, and said second bias current source are supplied by a regulated voltage. Advantageously, said plurality of capacitors within said first array are matched. Conveniently said plurality of capacitors within said second array are matched. According to another aspect of the present invention, there is provided a method for operating a ramp generator having an array of capacitors each controlled by a switch, comprising: resetting said ramp generator; enabling a current generator, said current generator charging at least one capacitor within said array; and controlling the state of one or more switches, wherein said switches are operable to connect/disconnect one or more of said capacitors within said array. Preferably, said resetting said ramp generator comprises: disabling said current generator; and controlling the state of a reset switch, said reset switch operable to connect the output of said ramp generator to a reference voltage. Conveniently, said controlling the state of one or more switches comprises supplying at least one control signal to each of said one or more switches. Advantageously, said resetting said ramp generator comprises: disabling said current generator; connecting the output of said ramp generator to a reference voltage; disconnecting the output of said ramp generator from said reference voltage; and enabling a second current generator for a predetermined period, said second current generator charging at least one capacitor in said array. According to another aspect of the present invention, there is provided an analog-to-digital converter, comprising: a ramp generator producing a ramp output, said ramp generator comprising: an array of capacitors each controlled by a switch operable to connect/disconnect one or more of said capacitors within said array, wherein said switches are responsive to one or more control signals; and a current source operable to charge at least one of said capacitors within said array; and a comparator producing a comparator output responsive to an analog input and said ramp output. Preferably, said array includes a reset switch and a non-switchable capacitance. Advantageously, said ramp generator further comprises a current source switch operable to disable said current source relative to said array. Conveniently, said capacitors within said ramp generator are matched. Preferably, said ramp generator further comprises: another current source for providing a bias function; and another current source switch operable to disable said another current source relative to said array. Advantageously, said ramp generator further comprises an operational amplifier, said operational amplifier having: a first input for receiving a reference voltage; a second input connected to at least one of an output of said current source, an output of said another current source, and a first side of said array; and an output connected to a second side of said array. Conveniently, said current source and said another current source are supplied by a regulated voltage. Preferably, said ramp output has a linear portion and a compressed portion, said compressed portion having a plurality of discreet segments, each segment being defined by one or more programmable breakpoints. According to another aspect of the present invention, there is provided a method for generating a ramp output using a ramp generator having an array of capacitors each controlled by a switch, said method comprising: resetting said ramp output to a constant level; changing said ramp output at a rate of change corresponding to a least significant bit; and changing said ramp output at another rate of change corresponding to another least significant bit. Preferably, the method further comprises repeating said changing said ramp output to another rate of change corresponding to a another least significant bit for a plurality of least significant bits. Advantageously, said resetting said ramp output at a constant level comprises: disabling a current source operable to charge at least one of said capacitors within said array; enabling the switches controlling said capacitors in said array; and driving the ramp output to a reference voltage. Conveniently, said resetting said ramp output at a constant level comprises: disabling a current source operable to charge at least one of said capacitors within said array; enabling the switches controlling said capacitors in said array; driving the ramp output to a reference voltage; and enabling a second current generator for a predetermined period, said current generator driving changing said ramp output by an offset value. Preferably, said changing said ramp output at a rate of change corresponding to a least significant bit comprises applying a control signal to one or more of said switches controlling said capacitors in said array at a programmable breakpoint. Advantageously, said changing said ramp output at another rate of change corresponding to another least significant bit comprises applying a confrol signal to one or more of said switches controlling said capacitors in said array at a programmable breakpoint. According to another aspect of the present invention, there is provided an analog-to-digital converter, comprising: a ramp generator, comprising: a falling ramp portion operable to produce a falling ramp signal; and a rising ramp portion operable to produce a rising ramp signal; and a conversion circuit, comprising: a differential amplifier operable to produce an amplified differential signal responsive to two or more input signals; a comparator operable to compare said amplified differential signal to the difference of said falling ramp signal and said rising ramp signal; and a logic circuit operable to produce a digital output responsive to an output of said comparator. Preferably, said falling ramp portion comprises a first operational amplifier and said rising ramp portion comprises a second operational amplifier, said first operational amplifier having: an input for receiving a first reference voltage; and an input connected to at least one of a first current source, a first bias current source, and a first side of a first capacitance; and an output connected to a second side of said first capacitance; said second operational amplifier having: an input for receiving a second reference voltage; an input connected to at least one of a second current source, a second bias current source, and a first side of a second capacitance; and an output connected to a second side of said second capacitance. Advantageously, said first capacitance comprises at least one of a first variable capacitance or a first array of capacitors wherein each capacitor is controlled by a switch and wherein said second capacitance comprises at least one of a second variable capacitance or a second array of capacitors wherein each capacitor is controlled by a switch. Conveniently, each of said switches for said first array is operable to connect/disconnect one or more of a said capacitors within said first array, each of said switches responsive to one or more control signals. Preferably, each of said switches for said second array is operable to connect/disconnect one or more of a said capacitors within said second array, each of said switches responsive to one or more control signals. Advantageously, at least one of said first current source, said first bias current source, said second current source, and said second bias current source are supplied by a regulated voltage. Conveniently, said plurality of capacitors within said first array are matched. Preferably, said plurality of capacitors within said second array are matched. Advantageously, said comparator is a two-stage AC coupled comparator comprising a differential comparator and an operational amplifier. Conveniently, said logic produces a digital output if the difference of said falling ramp signal and said rising ramp signal is greater than said amplified differential signal. One aspect of the invention relates to a ramp generator for an analog-todigital converter comprising an array of capacitors each controlled by a switch operable to connect/disconnect one or more of the capacitors within the array, wherein each of the switches is responsive to one or more control signals and a current source operable to charge at least one of the plurality of capacitors within the array. Another aspect of the invention relates to a ramp generator comprising first and second operational amplifiers. The first operational amplifier has an input for receiving a first reference voltage, an input connected to at least one of a first current source, a first bias current source, and a first side of a first array of capacitors each controlled by a switch, and an output connected to a second side of the first array. The second operational amplifier has an input for receiving a second reference voltage, an input connected to at least one of a second current source, a second bias current source, and a first side of a second array of capacitors each controlled by a switch, and an output connected to a second side of the second array. Another aspect of the invention relates to a method for operating a ramp generator having an array of capacitors each controlled by a switch comprising resetting the ramp generator, enabling a current generator, the current generator charging at least one capacitor within the array, and controlling the state of one or more switches, wherein the switches are operable to connect/disconnect one or more capacitors within the array. Another aspect of the invention relates to an analog-to-digital converter comprising a comparator and a ramp generator. The ramp generator is comprised of an array of capacitors each controlled by a switch operable to connect/disconnect one or more of a plurality of capacitors within the array, wherein the plurality of switches are responsive to one or more control signals and a current source operable to charge at least one of the plurality of capacitors within the array. Another aspect of the invention relates to a method for generating a ramp output using a ramp generator having an array of capacitors each controlled by a switch, the method comprising resetting the ramp output to a constant level, changing the ramp output at a rate of change corresponding to a least significant bit, and changing the ramp output at another rate of change corresponding to a another least significant bit. Another aspect of the invention relates to an analog-to-digital converter comprising a ramp generator and a conversion circuit. The ramp generator comprises first and second operational amplifiers, the first operational amplifier having an input for receiving a first reference voltage, an input connected to at least one of a first current source, a first bias current source, and a first side of a first array of capacitors each controlled by a switch, and an output connected to a second side of the first array, the output operable to carry a falling ramp signal, and the second operational amplifier having an input for receiving a second reference voltage, an input connected to at least one of a second current source, a second bias current source, and a first side of a second array of capacitors each controlled by a switch, and an output connected to a second side of the second array, the output operable to carry a rising ramp signal. The conversion circuit comprises a differential amplifier operable to produce an amplified differential signal responsive to two or more input signals, a comparator operable to compare the amplified differential signal to the difference of the falling ramp signal and the rising ramp signal, and a logic circuit operable to produce a digital output responsive to the comparison completed by the comparator. BRIEF DESCRIPTION OF THE DRAWINGS To enable the present invention to be easily understood and readily practiced, the present invention will now be described for purposes of illustration and not limitation, in connection with the following figures wherein: FIG. 1 is a simplified diagram of an analog-to-digital converter according to one embodiment. FIG. 2 is a simplified diagram of the ramp generator of FIG. 1 according to one embodiment. FIG. 3 is a timing diagram for the ramp generator of FIG. 2 according to one embodiment. FIG. 4 is a simplified diagram of the ramp generator of FIG. 1 according to an alternative embodiment. FIG. 5 is a timing diagram for the ramp generator of FIG. 4 according to an alternative embodiment. FIG. 6 illustrates a simplified diagram of the ramp generator of FIG. 1 according to another embodiment. FIG. 7 illustrates a simplified diagram of a ramp generator according to another embodiment. FIG. 7A illustrates a simplified diagram of a digital conversion circuit according to one embodiment. FIG. 8 is a simplified diagram of a feedback-type analog-to-digital converter according to the prior art. FIG. 9 is a simplified diagram of a dual-slope analog-to-digital converter according to the prior art. FIG. 10 is a simplified diagram of a flash analog-to-digital converter according to the prior art. FIG. 11 is a simplified diagram of a charge-redistribution analog-todigital converter according to the prior art. FIG. 12 is a simplified diagram of a ramp generator and comparator for a digital ramp analog-to-digital converter according to the prior art. FIG. 13 is a timing diagram for the ramp generator of FIG. 12 according to the prior art. DETAILED DESCRIPTION The detailed description sets forth specific embodiments which are described in sufficient detail to enable those skilled in the art to practice the present invention. It should be apparent to those skilled in the art that other embodiments may be utilized, and that logical, mechanical, and electrical changes may be made, while remaining within the scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the invention is defined only by the appended claims. FIG. 1 is a simplified diagram of an analog-to-digital converter (ADC) 5 according to one embodiment. ADC 5 includes a comparator 8 and a ramp generator 10. Comparator 8 receives an analog signal (VA) at a first input and the ramp voltage (Vramp) from the output of the ramp generator 10 at a second input. The ramp generator 10 utilizes an array of capacitors to produce the output signal Vrampin response to a reference voltage (Vref) and control signals (cfrl), among others. In operation, the ramp generator 10 is first reset such that Vrampis equal to Vref. The comparator 8 compares VAto Vramp. If VAis greater than Vramp, the output of the comparator 8 (Vout) is high and the control signals (cfrl) cause the ramp generator 10 to increase Vramp- If Vrampis greater than VA, Voutgoes low and the control signals (cfrl) cause the ramp generator 8 to stop increasing VrampThe digital equivalent of the input signal VAmay be determined from the ramp generator's 10 settings at the time that Voutgoes low. FIG. 2 is a simplified diagram of the ramp generator 10 of FIG. 1 according to one embodiment. Ramp generator 10 includes an array of capacitors (Ci, C2, C3, ... Cn.[iota]) each controlled by an associated switch (S1}S2, S3, ... Sn-1). Each switch is responsive to a corresponding control signal (cfrl), ctrl2, cfrl3, ... cfrln-[iota]). The ramp generator 10 may also include a reset switch (Srstresponsive to a reset confrol signal (crtlrst), a capacitor Cn(which in the current embodiment does not include a corresponding switch), and a current source 12. The current source 12 includes a corresponding switch (Sc) which is responsive to a control signal (ctrl ). The switch Scenables/disables (e.g., connects/disconnects) the current source 12 relative to the array, reset switch Srst, an capacitor Cn. In the current embodiment, the capacitors (C1, C2, C3, ... Cn-[iota], Cn) are matched. Using the current manufacturing techniques, the capacitors (C1, C2, C3, ... Cn-1, Cn) can be matched within approximately 0.05% of each other. Accordingly, an ADC incorporating the ramp generator 10 illustrated in FIG. 2 is more accurate than an ADC converter that incorporates prior art ramp generators (such as that shown in FIG. 12). It should be apparent to one skilled in the art that improved manufacturing techniques may lead to improved matching of the capacitors while remaining within the scope of the present invention. FIG. 3 is a timing diagram for the ramp generator 10 of FIG. 2 according to one embodiment. Operation begins when the ramp generator 10 is reset by disabling the current source 12 and activating the reset switch Srrst. Referring to FIG. 3, the ramp generator is reset when control signal ctrlcis low (thus opening switch Sc and disabling current source 12) and control signal cfrlrstis pulsed high (thus closing switch Srst)- In the current embodiment, the confrol signals ctrli, ctrl2, ctrl3, ... ctrln_1are all high at this time, thus capacitors (Cl5C2, C3, ... Cn-1are connected across the array. However, when switch Srstis closed, the ramp generator output (Vramp) is directly connected to Vrefsuch that the capacitors (C1, C2, C3, ... Cn-1, Cn) are effectively short circuited. After the ramp generator 10 is reset, control signal cfrlrstgoes low opening switch Srst. Control signal cfrlcthen goes high closing switch Scand enabling the current source 12. Current I flows from the current source 12 charging the capacitors (C1, C2, C3, ... Cn.1, Cn) and causing Vrampto rise at a constant rate, for example, as illustrated as the 1LSB (i.e., least significant bit) portion of the Vrampcurve in FIG. 3. The slope of the 1LSB portion of the Vramp curve can be defined as: Vramp= I*tj/C[tau], where CT= C\+ C2+ C3+ ... Cn-1+ C, and Cj = C[tau]/2; C, + C2= 2*CT/3; C, +C2+C3= 3*CT/4, etc. After t2seconds, control signal ctrll1goes low opening switch Si and disconnecting capacitor C\from the capacitor array. This changes the slope of the Vrampcurve at the breakpoint between the 1LSB and 2LSB portions of the Vrampcurve shown in FIG. 3. The slope of the 2LSB portion of the Vrampcurve can be defined as Vramp= I*(t2- t1)/(C[tau]- C,) or 2I*(t2- t1)/CT. After t2seconds, control signal ctrl2goes low opening switch S2and disconnecting capacitor C2from the capacitor array. This changes the slope of the Vrampcurve at the breakpoint between the 2LSB and 3LSB portions of the Vrampcurve shown in FIG. 3. The slope of the 3LSB portion of the Vramp curve can be defined as Vramp= I*(t3- t2)/(CT- C1- C2) or 3I*(t3- t2)/CT. At each break point, a capacitor is disconnected from the capacitor array changing the slope of the Vramp curve. The remaining slopes may be defined in a manner similar to that discussed above, for example, the slope of the nLSB can be defined as Vramp= I*(tn- tn-1)/(Cn) or nl*(tn- tn-1/CT. In the current embodiment, the ramp generator output curve has a linear portion and a compressed portion. The linear portion of the ramp may be defined as Vramp= Vref+ (I*t)/CT. The compressed portion includes a plurality of discreet segments. Each segment is defined by one or more programmable breakpoints. The location of the breakpoints may be programmed by setting the time intervals t1, t2, t3, ... tn-1as desired. The compressed portion of the ramp can be defined as Vramp = Vref+ (I*t,)/C[tau]+ 2I*(t2- t,)/C[tau] + 3I*(t3- t2)/CT+ .. + nI*(tn- tn-1)/CT- FIG. 4 is a simplified diagram of the ramp generator 10 of FIG. 1 according to an alternative embodiment. As discussed above in conjunction with FIG. 2, the ramp generator 10 of the alternative embodiment includes an array of capacitors (Cj, C2, C3, ... Cn-[iota]) and associated switches (Si, S2, S3, ... Sn.[iota]). Each switch is responsive to a corresponding confrol signal (ctrli, cfrl2,ctrl3, ... ctrln-i). The ramp generator 10 of the alternative embodiment may also include a reset switch (Sret) responsive to a reset control signal (crtlrst), a capacitor C" (which does not include a corresponding switch), and a current source 12. The current source 12 includes a corresponding switch (Sc) which is responsive to a control signal (cfrlc). The switch Sc enables/disables (e.g., connects/disconnects) the current source 12 relative to the array, reset switch Srst, and capacitor Cn. In the current embodiment, the capacitors (C1, C2, C3, ... Cn_1, Cn) are matched. Using the current manufacturing techniques, the capacitors (C1, C2, C3, ... Cn-1, Cn) can be matched within approximately 0.05% of each other. Accordingly, an ADC incorporating the ramp generator 10 illustrated in FIG. 4 is more accurate than an ADC converter that incorporates prior art ramp generators (such as that shown in FIG. 12). It should be apparent to one skilled in the art that improved manufacturing techniques may lead to improved matching of the capacitors while remaining within the scope of the present invention. The ramp generator 10 illustrated in FIG. 4 also includes a second current source 14. The current source 14 includes a corresponding switch (Sp) which is responsive to a control signal (ctrlp). The switch Spenables/disables (e.g., connects/disconnects) the current source 14 relative to the array, reset switch Srst, and capacitor Cn. The current source 14 may be used to provide a pedestal function (i.e., a bias function), for example, to offset-cancel dark currents that are present in the CMOS sensors used in imaging applications. Dark currents refer, for example, to currents that leak through the transistors comprising the CMOS sensors used in imaging applications. FIG. 5 is a timing diagram for the ramp generator 10 of FIG. 4 according to the alternative embodiment. Generally, the ramp generator 10 illustrated in FIG.4 functions in the same manner as the ramp generator 10 discussed above in conjunction with FIG. 2. However, in the alternative embodiment, the current source 14 is enabled for a time period tpafter the reset switch Srstis deactivated, but prior to current source 12 being enabled. Current IPflows from the current source 14 causing the output of the ramp generator (VramP) to increase from Vrefto Vref+ Vpe. It should be apparent to one skilled in the art that the value of Vpedis dependent upon Ipand tp. Thus, Vpedcan easily be controlled to offset any dark currents. FIG. 6 illustrates a simplified diagram of the ramp generator of FIG. 1 according to another embodiment. As discussed above in conjunction with FIG. 4, the ramp generator 10 of the current embodiment includes an array of capacitors (C1, C2, C3, ... Cn-1) and associated switches (Si, S2, S3, ... Sn-[iota]). Each switch is responsive to a corresponding control signal (ctrli, ctrl2, ctrl3, ... ctrln.1). The ramp generator 10 of the current embodiment also includes a reset switch (Sret) responsive to a reset control signal (crtlrst), a capacitor Cn(which in the current embodiment does not include a corresponding switch), a current source 12, and a current source 14. The current source 12 includes a corresponding switch (Sc) which is responsive to a control signal (ctrl ), whereas the current source 14 includes a corresponding switch (Sp) which is responsive to a control signal (ctrlp). The switches Sc and Sp enable/disable (e.g., connect/disconnect) the current sources 12 and 14, respectively, relative to the capacitor array. Unlike the current sources illustrated in FIGS. 2 and 4 which are illustrated as being supplied using VDD, the current sources illustrated in FIG. 6 are supplied by a regulated voltage supply (Vreg). Additionally, the ramp generator 10 illustrated in FIG. 6 includes an operational amplifier 16. In the current embodiment, the outputs of the current sources 12, 14, one side of capacitor Cn, one side of reset switch S^t, and one side of the capacitor array are connected to the negative input terminal of the op-amp 16. The other side of capacitor Cn, the other side of reset switch S^t, and the other side of the capacitor array are connected to the output of the opamp 16. A reference voltage (Vref) is connected to the positive input terminal of the op-amp 16. The op-amp 16 reduces the loading on the reference input voltage (Vref) and provides a constant voltage across, and eliminates voltage dependence of, the current sources 12, 14. It should be apparent to one skilled in the art that the ramp generator 10 illustrated in FIG. 6 is a single-slope ramp generator. It should further be apparent to one skilled in the art that the polarity of the ramp generator's output (i.e., Vra prising or falling) depends upon the direction of current flow through the current sources 12, 14. For example, in the configuration illustrated in FIG. 6, the ramp generator's output falls as current flows from Vregthrough current sources 12, 14. FIG. 7 illustrates a simplified diagram of the ramp generator 20 of FIG. 1 according to another embodiment. The ramp generator 20 may be used, for example, in combination with a digital conversion circuit (such as that illustrated in FIG. 7A) to comprise a differential column-parallel analog to digital converter. The analog-to-digital converter discussed in the current embodiment uses a differential conversion technique to obtain a 12-bit digital code from analog input signal, for example, from a CMOS sensor used in an imaging application. Referring to FIG. 7, the ramp generator 20 illustrated is a differential output ramp generator operable to produce two separate output voltages (i.e., Vramp_dnand Vramp up). In the current embodiment, the ramp generator may be divided into two halves. The first half, which may be referred to as a falling ramp portion 21, includes an op-amp 16(1), two current sources 12(1), 14(1), a variable capacitor CSi, and a reset switch Srst1. In the current embodiment, the outputs of the current sources 12(1), 14(1) and one side of the variable capacitor CSi, and one side of reset switch Srst1 are connected to the negative input terminal of the opamp 16(1). The other side of the variable capacitor CSi and the other side of the reset switch Srst1 are connected to the output of the op-amp 16(1). A reference voltage (Vref 1) is connected to the positive input terminal of the opamp 16(1). The falling ramp portion 21 of the ramp generator 20 produces the output signal Vramp dn. Initially, reset switch S^i is closed, thus discharging variable capacitor CS1. At the same time, the ramp output Vramp -dnis reset to Vrefh1- Reset switch Srst1 is then released once the output Vramp dnis settled. The current source 14(1) is then activated by closing switch SP1using control signal Cfrlp. The current source 14(1) supplies a current IP1which introduces an offset value at the output Vramp_dnto offset-cancel any dark currents, for example, generated by an input sensor. After switch SP1is opened, the ramping operation begins when current source 12(1) is activated by closing switch Sc1using control signal Cfrl1. The current source 12(1) supplies a current I1. The slope of the output ramp_up- Initially, reset switch Sret2i closed, thus discharging variable capacitor CS2. At the same time, the ramp output Vramp_up is reset to Vref_lo- Reset switch Sret2is then released once the output Vramp_up is settled. The current source 14(2) is then activated by closing switch Sp2using control signal Cfrlp. The ramp is constant up to the point when the variable capacitor CS, is adjusted at the required break point by switching out a fraction of the capacitor. The ramp output Vramp_dncan be defined by the following equation: Vramp_dn= Vref_h[iota]- (Ipi* tP)/CS1 - (Ii * t1)/CS1 - 21, * (t2-t1)/CSl - 31l* (t3-t2)/CS, - ... - nll* (tn-tn-1)/CS1. It should be noted that the variable capacitor CS, may be implemented using an array of capacitors, for example, capacitors (C11, C12, C13, .. C,n-1) and associated switches (S11, S12, S13, ... S1n-1), each switch responsive to a corresponding control signal (ctrln, ctrl,2, ct[tau]l13, ... cfrlln-1). Accordingly, one skilled in the art should recognize that the falling ramp portion 21 of the ramp generator 20 illustrated in FIG. 7 may be constructed and operated in a manner similar to the ramp generator 10 discussed above in conjunction with FIG. 6. The second half, which may be referred to as a rising ramp portion 22, includes an op-amp 16(2), two current sources 12(2), 14(2), a variable capacitor CS2, and a reset switch Srst2- In the current embodiment, the outputs of the current sources 12(2), 14(2), one side of the variable capacitor CS2, and one side of reset switch Srst2are connected to the negative input terminal of the opamp 16(2). The other side of the variable capacitor CS2and the other side of reset switch Sret2are connected to the output of the op-amp 16(2). A reference voltage (Vref lo) is connected to the positive input terminal of the op-amp 16(2). The rising ramp portion 22 of the ramp generator 20 produces the output signal V - current source 14(2) supplies a current IP2which introduces an offset value at the output Vra p_upto offset-cancel any dark current, for example, generated by an input sensor. After switch SP2is opened, the ramping operation begins when current source 12(2) is activated by closing switch Sc2using control signal Ctrl1. The current source 12(2) supplies a current I2. The slope of the output ramp is constant up to the point when the variable capacitor CS2is adjusted at the required break point by switching out a fraction of the capacitor. The ramp output Vramp_upcan be defined by the following equation: Vramp_up≤>Vreflp+ (IP2* tP)/CS2+ (I2* tl)/CS2+ 2I2* (t2-t,)/CS2+ 3I2* (t3-t2)/CS2+ ... + nl2* (tn-tn-l)/CS2. It should be noted that the variable capacitor CS2may be implemented using an array of capacitors, for example, capacitors (C21, C22, C23, ... C2n-,) and associated switches (S21, S22, S23, ... S2n.1), each switch responsive to a corresponding confrol signal (cfrl21, ctrl22, ctrl23, ... ctrl2n-1). Accordingly, one skilled in the art should recognize that the rising ramp portion 22 of the ramp generator 20 illustrated in FIG. 7 may be constructed and operated in a manner similar to the ramp generator 10 discussed above in conjunction with FIG. 6, with the exception that for the rising ramp portion 22, the non-inverting input of Op-Amp2 is connected to a low reference voltage (i.e., Vref_,o) and the current sources (i.e., 12(2), 14(2)) are supplied by a sinking regulated supply (i.e., Vreg2). It should further be noted that the falling ramp portion 21 and the rising ramp portion 22 may be operated individually or simultaneously while remaining within the scope of the present invention. Referring now to FIG. 7A, the differential conversion circuit 200 receives the output signals Vramp uand Vramp upfrom the differential ramp generator 20 illustrated in FIG. 7. The conversion circuit 200 includes a differential amplifier 216, a two-stage AC-coupled comparator comprised of a differential comparator 234 and a second amplifier 246, latching/RAM logic 248, switches 202, 204, 214, 218, 220, 222, 232, 236, 242, 244, capacitors 208, 210, 224, 226, 228, 230, 238, 240 and variable capacitors 206, 212. Operation of the differential column-parallel ADC is generally as follows. Analog signals Vcolrand Vco,s(e.g., from a CMOS image sensor) are input to the differential amplifier 216. The difference between Vcolrand Vcolsis amplified by the differential amplifier 216. This amplified differential signal is stored between nodes Nr and Ns. Simultaneously, the two-stage AC-coupled comparator 234, 246 is primed for action by biasing the inputs and outputs at ~VDD/2 and Vref. This biasing is accomplished using switches 232, 236, and 244. During the analog-to-digital conversion, the amplified differential signal stored at nodes Nr and Ns is compared to the outputs from the differential ramp generator (i.e., Vramp dnand Vramp_up). The latching/RAM logic 248 generates a 12-bit code in response to the output of the two-stage AC-coupled comparator. In the current embodiment, for example, the latching/RAM logic 248 generates a 12-bit code if the differential ramp signal is greater than the amplified differential signal. It should be apparent to those of ordinary skill in the art that equivalent logic or physical circuits may be constructed using alternate logic elements while remaining within the scope of the present invention. It should further be recognized that the above-described embodiments of the invention are intended to be illustrative only. Numerous alternative embodiments may be devised by those skilled in the art without departing from the scope of the following claims. When used in this specification and claims, the terms "comprises" and "comprising" and variations thereof mean that the specified features, steps or integers are included. The terms are not to be interpreted to exclude the presence of other features, steps or components. The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

Claims

1. A ramp generator for an analog-to-digital converter, comprising: an array of capacitors each controlled by a switch operable to connect/disconnect one or more of said capacitors within said array, wherein each of said switches is responsive to one or more control signals; and a current source operable to charge at least one of said plurality of capacitors within said array.

2. The ramp generator of claim 1 further comprising a reset switch and a nonswitchable capacitance.

3. The ramp generator of claim 1 further comprising a current source switch operable to disable said current source relative to said array. The ramp generator of claim 1 wherein said capacitors are matched.

5. The ramp generator of claim 3 further comprising: another current source for providing a bias function; and another current source switch operable to disable said another current source relative to said array.

6. The ramp generator of claim 5 further comprising an operational amplifier, said operational amplifier having: a first input for receiving a reference voltage; a second input connected to at least one of an output of said current source, an output of said another current source, and a first side of said array; and an output connected to a second side of said array.

7. The ramp generator of claim 6 wherein said current source and said another current source are supplied by a regulated voltage.

8. The ramp generator of claim 1 wherein the ramp generator is operable to produce an output having a linear portion and a compressed portion, wherein said compressed portion has a plurality of discreet segments, each segment being defined by one or more programmable breakpoints.

9. A ramp generator, comprising: a falling ramp portion; and a rising ramp portion.

10. The ramp generator of claim 9 wherein said falling ramp portion comprises a first operational amplifier and said rising ramp portion comprises a second operational amplifier, said first operational amplifier having: an input for receiving a first reference voltage; and an input connected to at least one of a first current source, a first bias current source, and a first side of a first capacitance; and an output connected to a second side of said first capacitance; said second operational amplifier having: an input for receiving a second reference voltage; an input connected to at least one of a second current source, a second bias current source, and a first side of a second capacitance; and an output connected to a second side of said second capacitance.

11. The ramp generator of claim 10 wherein said first capacitance comprises at least one of a first variable capacitance or a first array of capacitors wherein each capacitor is controlled by a switch and wherein said second capacitance comprises at least one of a second variable capacitance or a second array of capacitors wherein each capacitor is controlled by a switch.

12. The ramp generator of claim 11 wherein each of said switches for said first array is operable to connect/disconnect one or more of a said capacitors within said first array, each of said switches responsive to one or more confrol signals.

13. The ramp generator of claim 11 wherein each of said switches for said second array is operable to connect/disconnect one or more of a said capacitors within said second array, each of said switches responsive to one or more control signals.

14. The ramp generator of claim 10 wherein at least one of said first current source, said first bias current source, said second current source, and said second bias current source are supplied by a regulated voltage.

15. The ramp generator of claim 11 wherein said plurality of capacitors within said first array are matched.

16. The ramp generator of claiml l wherein said plurality of capacitors within said second array are matched.

17. A method for operating a ramp generator having an array of capacitors each controlled by a switch, comprising: resetting said ramp generator; enabling a current generator, said current generator charging at least one capacitor within said array; and controlling the state of one or more switches, wherein said switches are operable to connect/disconnect one or more of said capacitors within said array.

18. The method of claim 17 wherein said resetting said ramp generator comprises: disabling said current generator; and controlling the state of a reset switch, said reset switch operable to connect the output of said ramp generator to a reference voltage.

19. The method of claim 17 wherein said controlling the state of one or more switches comprises supplying at least one control signal to each of said one or more switches. 20. The method of claim 17 wherein said resetting said ramp generator comprises: disabling said current generator; connecting the output of said ramp generator to a reference voltage; disconnecting the output of said ramp generator from said reference voltage; and enabling a second current generator for a predetermined period, said second current generator charging at least one capacitor in said array.

21. An analog-to-digital converter, comprising: a ramp generator producing a ramp output, said ramp generator comprising: an array of capacitors each controlled by a switch operable to connect/disconnect one or more of said capacitors within said array, wherein said switches are responsive to one or more control signals; and a current source operable to charge at least one of said capacitors within said array; and a comparator producing a comparator output responsive to an analog input and said ramp output.

22. The analog-to-digital converter of claim 21 wherein said array includes a reset switch and a non-switchable capacitance.

23. The analog-to-digital converter of claim 21 wherein said ramp generator further comprises a current source switch operable to disable said current source relative to said array.

24. The analog-to-digital converter of claim 21 wherein said capacitors within said ramp generator are matched.

25. The analog-to-digital converter of claim 21 wherein said ramp generator further comprises: another current source for providing a bias function; and another current source switch operable to disable said another current source relative to said array.

26. The analog-to-digital converter of claim 25 wherein said ramp generator further comprises an operational amplifier, said operational amplifier having: a first input for receiving a reference voltage; a second input connected to at least one of an output of said current source, an output of said another current source, and a first side of said array; and an output connected to a second side of said array.

27. The analog-to-digital converter of claim 25 wherein said current source and said another current source are supplied by a regulated voltage.

28. The analog-to-digital converter of claim 21 wherein said ramp output has a linear portion and a compressed portion, said compressed portion having a plurality of discreet segments, each segment being defined by one or more programmable breakpoints.

29. A method for generating a ramp output using a ramp generator having an array of capacitors each controlled by a switch, said method comprising: resetting said ramp output to a constant level; changing said ramp output at a rate of change corresponding to a least significant bit; and changing said ramp output at another rate of change corresponding to another least significant bit.

30. The method of claim 29 further comprising repeating said changing said ramp output to another rate of change corresponding to a another least significant bit for a plurality of least significant bits.

31. The method of claim 29 wherein said resetting said ramp output at a constant level comprises: disabling a current source operable to charge at least one of said capacitors within said array; enabling the switches controlling said capacitors in said array; and driving the ramp output to a reference voltage.

32. The method of claim 29 wherein said resetting said ramp output at a constant level comprises: disabling a current source operable to charge at least one of said capacitors within said array; enabling the switches controlling said capacitors in said array; driving the ramp output to a reference voltage; and enabling a second current generator for a predetermined period, said current generator driving changing said ramp output by an offset value.

33. The method of claim 29 wherein said changing said ramp output at a rate of change corresponding to a least significant bit comprises applying a control signal to one or more of said switches controlling said capacitors in said array at a programmable breakpoint.

34. The method of claim 29 wherein said changing said ramp output at another rate of change corresponding to another least significant bit comprises applying a control signal to one or more of said switches controlling said capacitors in said array at a programmable breakpoint.

35. An analog-to-digital converter, comprising: a ramp generator, comprising: a falling ramp portion operable to produce a falling ramp signal; and a rising ramp portion operable to produce a rising ramp signal; and a conversion circuit, comprising: a differential amplifier operable to produce an amplified differential signal responsive to two or more input signals; a comparator operable to compare said amplified differential signal to the difference of said falling ramp signal and said rising ramp signal; and a logic circuit operable to produce a digital output responsive to an output of said comparator.

36. The ramp generator of claim 35 wherein said falling ramp portion comprises a first operational amplifier and said rising ramp portion comprises a second operational amplifier, said first operational amplifier having: an input for receiving a first reference voltage; and an input connected to at least one of a first current source, a first bias current source, and a first side of a first capacitance; and an output connected to a second side of said first capacitance; said second operational amplifier having: an input for receiving a second reference voltage; an input connected to at least one of a second current source, a second bias current source, and a first side of a second capacitance; and an output connected to a second side of said second capacitance.

37. The ramp generator of claim 36 wherein said first capacitance comprises at least one of a first variable capacitance or a first array of capacitors wherein each capacitor is controlled by a switch and wherein said second capacitance comprises at least one of a second variable capacitance or a second array of capacitors wherein each capacitor is controlled by a switch.

38. The ramp generator of claim 37 wherein each of said switches for said first array is operable to connect/disconnect one or more of a said capacitors within said first array, each of said switches responsive to one or more confrol signals.

39. The ramp generator of claim 37 wherein each of said switches for said second array is operable to connect/disconnect one or more of a said capacitors within said second array, each of said switches responsive to one or more confrol signals.

40. The ramp generator of claim 36 wherein at least one of said first current source, said first bias current source, said second current source, and said second bias current source are supplied by a regulated voltage.

41. The ramp generator of claim 37 wherein said plurality of capacitors within said first array are matched.

42. The ramp generator of claim 37 wherein said plurality of capacitors within said second array are matched.

43. The ramp generator of claim 35 wherein said comparator is a two-stage AC coupled comparator comprising a differential comparator and an operational amplifier.

44. The ramp generator of claim 35 wherein said logic produces a digital output if the difference of said falling ramp signal and said rising ramp signal is greater than said amplified differential signal.

45. A ramp generator substantially as hereinbefore described with reference to and as shown in Figures 1 to 7 of the accompanying drawings.

46. An analog-to-digital converter substantially as hereinbefore described with reference to and as shown in Figures 1 to 7 of the accompanying drawings.

47. A method for generating a ramp output substantially as hereinbefore described with reference to Figures 1 to 7 of the accompanying drawings.

48. A method for operating a ramp generator substantially as hereinbefore described with reference to Figures 1 to 7 of the accompanying drawings.

49. Any novel feature or combination of features disclosed herein. 1