US20070146181A1 - Digital background calibration for time-interlaced analog-to-digital converters - Google Patents
Digital background calibration for time-interlaced analog-to-digital converters Download PDFInfo
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- US20070146181A1 US20070146181A1 US11/315,640 US31564005A US2007146181A1 US 20070146181 A1 US20070146181 A1 US 20070146181A1 US 31564005 A US31564005 A US 31564005A US 2007146181 A1 US2007146181 A1 US 2007146181A1
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- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03M—CODING; DECODING; CODE CONVERSION IN GENERAL
- H03M1/00—Analogue/digital conversion; Digital/analogue conversion
- H03M1/10—Calibration or testing
- H03M1/1004—Calibration or testing without interrupting normal operation, e.g. by providing an additional component for temporarily replacing components to be tested or calibrated
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- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03M—CODING; DECODING; CODE CONVERSION IN GENERAL
- H03M1/00—Analogue/digital conversion; Digital/analogue conversion
- H03M1/12—Analogue/digital converters
- H03M1/1205—Multiplexed conversion systems
- H03M1/121—Interleaved, i.e. using multiple converters or converter parts for one channel
- H03M1/1215—Interleaved, i.e. using multiple converters or converter parts for one channel using time-division multiplexing
Definitions
- the present invention is directed, in general, to analog-to-digital conversion and, more specifically, to an apparatus and method for calibrating time-interlaced analog-to-digital converters.
- ADCs analog-to-digital converters
- One way of achieving such ADCs is to employ a time-interleaved architecture.
- Time-interleaved architectures provide a benefit of increased sampling rate for an analog signal and may employ a broad spectrum of ADC technologies. However, this benefit is usually achieved at the expense of both larger semiconductor die area and power consumption.
- Time-interleaved ADCs also generally provide conversion-related errors due to mismatches among channel ADCs that occur in the areas of offset, gain and timing. These mismatches cause spurious components in the spectrum of the TIADC thereby generally degrading the signal-to-noise-and-distortion ratio (SNDR) of the TIADC.
- SNDR signal-to-noise-and-distortion ratio
- timing mismatch errors are a primary limiting factor and give rise to higher noise power in the overall output.
- Such timing mismatches have generally two different aspects. These include random sampling jitter and fixed periodic timing-skew among different channels. The use of sample-and-hold amplifiers reduces timing mismatch, but usually limits the overall throughput speed of the TIADC.
- the background calibrator includes a TIADC having a parallel array of time-interleaved main signal processors, each main signal processor including an ADC connected to a corresponding output FIR filter.
- the background calibrator also includes an auxiliary signal processor having an ADC connected to at least one corresponding output FIR filter.
- the background calibrator further includes a timing calibration circuit, wherein the timing calibration circuit is configured to select one of the main signal processors, exchange the auxiliary signal processor with the selected main signal processor in the TIADC, and connect the selected main signal processor to the timing calibration circuit.
- the timing calibration circuit is further configured to reduce a timing mismatch of the selected main signal processor based on interpolation quantities and timing mismatch operations defined for calibration of the auxiliary signal processor.
- the present invention provides a method for calibrating a TIADC.
- the method includes selecting a main signal processor, which includes an ADC connected to a corresponding output FIR filter, from a parallel array of time-interleaved main signal processors in the TIADC.
- the method also includes exchanging an auxiliary signal processor, which includes an ADC connected to at least one corresponding output FIR filter, with the selected main signal processor and connecting the selected main signal processor for calibration.
- the method further includes reducing a timing mismatch of the selected main signal processor based on interpolation quantities and timing mismatch operations defined for calibration of the auxiliary signal processor.
- FIG. 1A illustrates a block diagram of an embodiment of a background calibrator constructed in accordance with the principles of the present invention
- FIG. 1B illustrates an alternative connection of the background calibrator of FIG. 1A constructed in accordance with the principles of the present invention
- FIG. 2 illustrates an embodiment of an interpolation chart constructed in accordance with the principles of the present invention
- FIG. 3 illustrates a power spectral density diagram associated with a TIADC as was discussed with respect to FIGS. 1A and 1B ;
- FIG. 4 illustrates an SNDR chart showing uncalibrated and calibrated responses for a TIADC system constructed in accordance with the principles of the present invention
- FIG. 5 illustrates a flow diagram of an embodiment of a method for calibrating a TIADC carried out in accordance with the principles of the present invention.
- the background calibrator 100 includes a time-interleaved analog-to-digital converter (TIADC) 105 coupled to input and output switch banks 110 , 115 , an auxiliary signal processor 120 also coupled to the input and output switch banks 110 , 115 and a timing calibration circuit 125 coupled to the input and output switch banks 110 , 115 and a filter coefficient switch 135 .
- TIADC time-interleaved analog-to-digital converter
- the TIADC 105 includes a parallel array of four main signal processors 105 1 - 105 4 , although generally, any plurality of such processors may be employed as appropriate to a particular application.
- Each of the main signal processors 105 1 - 105 4 includes a main analog-to-digital converter (ADC) and a corresponding output FIR filter (i.e., ADC 1 -ADC 4 and corresponding FIR 1 -FIR 4 ).
- the auxiliary signal processor 120 includes a calibration ADC and four calibration output FIR filters (i.e., ADC CAL and corresponding FIR CAL1-4 ).
- Each first input pole of the input switch bank 110 is connected to an analog input signal bus 106 , and each second input pole of the input switch bank 110 is connected to a ramp input signal bus 107 .
- each first output pole of the output switch bank 115 is connected to a digital output signal bus 116 .
- Each second output pole of the output switch bank 115 associated with the main signal processors 105 1 - 105 4 , is connected to a mismatch calculation input 117 of the timing calibration circuit 125 .
- a second output pole of the output switch bank 115 associated with the auxiliary signal processor 120 , is connected to an interpolation input 118 of the timing calibration circuit 125 .
- the timing calibration circuit 125 includes a first register 126 containing initial digitized data stored in a first register ( 1 ), a digital interpolation module 127 , a register array 128 containing interpolated data stored in a second register ( 2 ), a third register ( 3 ) and a fourth register ( 4 ) and a mismatch calculation module 129 that provides a mismatch calculation output 130 connected to the filter coefficient switch 135 .
- the background calibrator 100 provides estimation and compensation of a fixed periodic timing mismatch among the parallel array of four main signal processors 105 1 - 105 4 .
- estimation of timing mismatch requires the auxiliary signal processor 120 employing the additional calibration ADC CAL and the four calibration output FIR filters FIR CAL1-4 .
- the calibration filters FIR CAL1-4 are employed to correct the timing errors.
- the proposed calibration technique can significantly improve the signal-to-noise-and-distortion ratio (SNDR) performance for the TIADC 105 .
- SNDR signal-to-noise-and-distortion ratio
- the background calibrator 100 employs switch positions shown in FIG. 1A for the input and output switch banks 110 , 115 and the filter coefficient switch 135 to provide needed digital interpolation and compensation parameters.
- an input ramp signal is connected to the compensation ADC cal and corresponding filter FIR CAL1 , while employing a first sampling clock clk 1 , wherein digitized calibration data are stored into the first register ( 1 ).
- the digital interpolation circuit 127 follows the first register ( 1 ) and employs the digitized calibration data of register ( 1 ) to estimate the next quantized data corresponding to different-phase sampling clocks (i.e., second, third and fourth sampling clocks clk 2-4 ).
- the four sampling clocks clk 1-4 approximately occupy zero, 90, 180 and 270 degrees of phase shift in the sampling period for the four stages of TIADC 105 .
- FIG. 2 illustrated is an embodiment of an interpolation chart, generally designated 200 , constructed in accordance with the principles of the present embodiment.
- the interpolation chart 200 shows digital data samples and estimates for the calibration ADC CAL having an input ramp signal and employing the four sampling clocks clk 1-4 .
- a mean function associated with the digital interpolation module 127 may be employed to estimate the digital data associated with the calibration ADC cal using the second, third and fourth sampling clocks clk 2-4 .
- reference data stored in register ( 1 ) is employed while sequentially using sampling clocks clk 2-4 for the calibration ADC cal to sample the ramp input signal 105 . This action yields interpolation values y 3,1 at t 3,1 , y 2,1 at t 2,1 and y 4,1 at t 4,1 , respectively.
- the interpolated data is then stored in the register array 128 employing the second, third and fourth registers ( 2 )-( 4 ).
- the interpolated data are regarded as ideal sampled data of the calibration ADC cal and provide sampled data for ideal clock positions of zero, 90, 180 and 270 degrees having no timing errors.
- the interpolated data in the registers ( 1 )-( 4 ) are used for calibration as reference data.
- the calibration ADC cal samples the input ramp signal employing the second, third and fourth sampling clocks clk 2-4 .
- Output data are compared with the reference data in registers ( 2 )-( 4 ) and adjustments are made to the coefficients of FIR CAL2-4 employing the mismatch calculation module 129 and the mismatch calculation output 130 .
- the errors between actual sampled data and the reference data in registers ( 1 )-( 4 ) come from timing errors.
- the timing errors may be calculated employing the mismatch calculation module 129 according to equation (2) below.
- ⁇ ⁇ ⁇ t i , m y i , m * - y i , m S , ( 2 )
- S is the slope of the ramp signal
- y i,m * is the measured digital data
- y i,m is the interpolated register data. Adjustments are made to the coefficients of FIR CAL2-4 until the sample timing errors are reduced below a preselected threshold.
- FIR filters typically introduce delays of many sample periods. However, FIR filters may also introduce fractional sample-period delays when sample time errors between interleaved main signal processors exist. These fractional delays may be both positive and negative depending on the filter coefficients employed. This principle is employed to compensate for timing mismatches between the main signal processors of the TIADC 105 . A more detailed explanation of employing FIR filters to compensate for timing errors may be seen in “A 10-b 120-Msample/s Time-Interleaved Analog-to-Digital Converter With Digital Background Calibration”, by Shafiq M. Jamal, et al., IEEE Journal Of Solid-State Circuits, Vol. 37, No. 12, December 2002, which is incorporated herein by reference.
- FIG. 1B illustrated is an alternative connection of the background calibrator of FIG. 1A , generally designated 120 , constructed in accordance with the principles of the present invention.
- the background calibrator 120 shows the auxiliary signal processor 120 being substituted for one of the main signal processors (the second main signal processor 105 2 , as exemplary) while it is being calibrated.
- the auxiliary signal processor 120 is connected between the analog input signal bus 106 and the digital output signal bus 116 while employing the second sampling clock clk 2 to allow normal operation of the TIADC 105 .
- the second main signal processor 105 2 is connected to the ramp input signal bus 107 and the mismatch calculation 117 to allow its calibration.
- a final calibration is carried out for each of the main ADC 1 -ADC 4 (i.e., main ADC i ).
- This calibration phase again employs the timing calibration circuit 125 as discussed with respect to FIG. 1A .
- the relative timing mismatches of the four main ADCs relative to the references of the timing calibration circuit 125 are reduced below a preselected threshold by adjusting the coefficients of the corresponding FIR filter (i.e., FIR i ) associated with each main ADC i .
- the input and output connections may again be employed as depicted in FIG. 1A .
- FIG. 3 illustrated is a power spectral density diagram, generally designated 300 , associated with a TIADC as was discussed with respect to FIGS. 1A and 1B .
- the diagram 300 includes an uncalibrated output power spectral density (PSD) 305 and a calibrated output PSD 310 .
- PSD output power spectral density
- the uncalibrated output PSD 305 shows spurious responses 305 a , 305 b , 305 c associated with timing errors before calibration.
- the calibrated output PSD 310 shows the output spectrum after calibration as carried out in accordance with the principles of the present invention.
- a TIADC using four time-interleaved channels of 12-bit ADCs was employed to demonstrate the benefits of compensation provided by an embodiment for periodic timing-mismatch conditions.
- the number of filter taps needed in the corresponding FIR filters is mainly dependent on the required ADC resolution and the amount of sample timing error encountered.
- a FIR filter having 45-taps is employed for timing error correction in each of the main ADCs. This accommodates a fixed timing mismatch error of about one percent of sampling time among the time-interleaved phase generation paths.
- the uncalibrated output PSD 305 shows the output spectrum of the TIADC without timing calibration.
- the calibrated output PSD 310 shows the benefits of calibration. The magnitude of the spurious response has been suppressed by about 50 dB, which is enough suppression to provide a clear spectrum for the 12-bit TIADC.
- the SNDR chart 400 includes an uncalibrated SNDR curve 405 and a calibrated SNDR curve 410 for normalized timing errors.
- the SNDR may be seen to drop exponentially as timing error increases thereby demonstrating an SNDR of only about 30 dB at a normalized timing error of about 12 percent.
- the calibrated SNDR curve 410 showing the effect of calibration, provides an almost flat SNDR of greater than 70 dB for a normalized timing error up to about 12.
- FIG. 5 illustrated is a flow diagram of an embodiment of a method for calibrating a TIADC, generally designated 500 , which may be used, for example, for calibrating the TIADC of FIG. 1A or FIG. 1B .
- the method 500 is for use with a parallel array of main signal processing ADCs and corresponding FIR filters employed in a time-interleaved manner. Additionally, the method uses an auxiliary signal processor having an ADC that is also connected to a parallel plurality of output FIR filters, wherein each of these output FIR filters corresponds to one of the main output FIR filters.
- the auxiliary signal processor may be employed to achieve a background calibration mode of operation.
- the method 500 starts in a step 505 .
- the method 500 involves applying an input ramp signal to the auxiliary signal processor that employs a first sampling clock as a reference clock. This provides samples of the input ramp voltage that serve as reference samples for interpolation.
- the method 500 involves providing sample values that interpolate between two of the reference samples and correspond to ideal time-interleaved sampling times for the remaining number of sampling clocks employed in the TIADC. These interpolated sample values may be employed as reference data, because the ideal time-interleaved sampling times have no timing errors with respect to the first sampling clock.
- the auxiliary signal processor then employs each of the remaining sampling clocks, in turn, to provide actual sample values of the input ramp signal.
- the method 500 involves determining a timing mismatch between the actual sample values and the interpolated sample values for each of the remaining sampling clocks. Then, in a first decisional step 525 , the method 500 involves further determining whether the timing mismatch, which was determined in the step 520 , is below a preselected threshold. If the timing mismatch is not below the preselected threshold, the method 500 includes reducing the timing mismatch by modifying filter coefficients of the output FIR filter being employed with the auxiliary signal processor for the selected remaining sampling clock. Then, the method 500 includes returning to the step 510 and repeating the steps until the timing mismatches for all of the remaining sampling clocks are below the preselected threshold.
- the method 500 involves exchanging the calibrated auxiliary signal processor for a selected main signal processor, in a step 535 .
- the auxiliary signal processor employs the sampling clock and calibrated output FIR filter that corresponds to the sampling clock and output FIR filter of the selected main signal processor in the exchange. This allows the selected main signal processor to be calibrated in a background mode while the TIADC continues to be fully functional.
- the method 500 includes applying the input ramp signal to the selected main signal processor, while employing its regular sampling clock and output FIR filter, to provide an actual sample value.
- the method 500 involves determining the timing mismatch between the actual sample value and the corresponding interpolated value.
- the method 500 further involves determining whether the timing mismatch is below the preselected threshold. If it is not below the preselected threshold, the timing mismatch is reduced by modifying the filter coefficients of the selected output FIR filter.
- the method 500 involves returning to the step 540 and executing the steps 540 - 550 until the remaining timing mismatch is below the preselected threshold.
- the method 500 involves performing a third decisional step 560 to determine whether the last main signal processor has been selected for calibration. If the last main signal processor has not been selected, the previously selected and now calibrated main signal processor is returned to the TIADC, and the method 500 involves returning to the step 535 .
- the calibrated auxiliary signal processor is exchanged for another selected main signal processor, which is to be calibrated. This sequence continues until it is determined in the third decisional step 560 that the last main signal processor has been calibrated, wherein the method 500 ends in a step 565 .
- TIADC time-interleaved analog-to-digital converter
- Advantages include a reduction of fixed periodic timing mismatch effects in the TIADC.
- the compensation utilizes FIR filters to correct the timing errors.
- the signal-to-noise-and-distortion ratio (SNDR) performance for a 12-bit TIADC system was shown to be significantly enhanced by the calibrator and the method.
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Abstract
Description
- The present invention is directed, in general, to analog-to-digital conversion and, more specifically, to an apparatus and method for calibrating time-interlaced analog-to-digital converters.
- The increasing interest in higher data rate communication applications requires high-speed and high-resolution analog-to-digital converters (ADCs). One way of achieving such ADCs is to employ a time-interleaved architecture. Time-interleaved architectures provide a benefit of increased sampling rate for an analog signal and may employ a broad spectrum of ADC technologies. However, this benefit is usually achieved at the expense of both larger semiconductor die area and power consumption.
- Time-interleaved ADCs (TIADCs) also generally provide conversion-related errors due to mismatches among channel ADCs that occur in the areas of offset, gain and timing. These mismatches cause spurious components in the spectrum of the TIADC thereby generally degrading the signal-to-noise-and-distortion ratio (SNDR) of the TIADC. In particular, timing mismatch errors are a primary limiting factor and give rise to higher noise power in the overall output. Such timing mismatches have generally two different aspects. These include random sampling jitter and fixed periodic timing-skew among different channels. The use of sample-and-hold amplifiers reduces timing mismatch, but usually limits the overall throughput speed of the TIADC.
- Accordingly, what is needed in the art is an enhanced way to correct timing errors inherent in the use of multiple ADCs in a time-interleaved architecture.
- To address the above-discussed deficiencies of the prior art, various embodiments provide background calibrators. In one embodiment, the background calibrator includes a TIADC having a parallel array of time-interleaved main signal processors, each main signal processor including an ADC connected to a corresponding output FIR filter. The background calibrator also includes an auxiliary signal processor having an ADC connected to at least one corresponding output FIR filter. Additionally, the background calibrator further includes a timing calibration circuit, wherein the timing calibration circuit is configured to select one of the main signal processors, exchange the auxiliary signal processor with the selected main signal processor in the TIADC, and connect the selected main signal processor to the timing calibration circuit.
- In an alternative embodiment, the timing calibration circuit is further configured to reduce a timing mismatch of the selected main signal processor based on interpolation quantities and timing mismatch operations defined for calibration of the auxiliary signal processor.
- In another aspect, the present invention provides a method for calibrating a TIADC. The method includes selecting a main signal processor, which includes an ADC connected to a corresponding output FIR filter, from a parallel array of time-interleaved main signal processors in the TIADC. The method also includes exchanging an auxiliary signal processor, which includes an ADC connected to at least one corresponding output FIR filter, with the selected main signal processor and connecting the selected main signal processor for calibration. In an alternative embodiment, the method further includes reducing a timing mismatch of the selected main signal processor based on interpolation quantities and timing mismatch operations defined for calibration of the auxiliary signal processor.
- The foregoing has outlined preferred and alternative features of various embodiments, so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features of the embodiments will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiments as a basis for designing or modifying other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention.
- For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:
-
FIG. 1A illustrates a block diagram of an embodiment of a background calibrator constructed in accordance with the principles of the present invention; -
FIG. 1B illustrates an alternative connection of the background calibrator ofFIG. 1A constructed in accordance with the principles of the present invention; -
FIG. 2 illustrates an embodiment of an interpolation chart constructed in accordance with the principles of the present invention; -
FIG. 3 illustrates a power spectral density diagram associated with a TIADC as was discussed with respect toFIGS. 1A and 1B ; -
FIG. 4 illustrates an SNDR chart showing uncalibrated and calibrated responses for a TIADC system constructed in accordance with the principles of the present invention; and -
FIG. 5 illustrates a flow diagram of an embodiment of a method for calibrating a TIADC carried out in accordance with the principles of the present invention. - Referring initially to
FIG. 1A , illustrated is a block diagram of an embodiment of a background calibrator, generally designated 100, constructed in accordance with the principles of the present invention. Thebackground calibrator 100 includes a time-interleaved analog-to-digital converter (TIADC) 105 coupled to input andoutput switch banks auxiliary signal processor 120 also coupled to the input andoutput switch banks timing calibration circuit 125 coupled to the input andoutput switch banks filter coefficient switch 135. - In the illustrated embodiment, the TIADC 105 includes a parallel array of four main signal processors 105 1-105 4, although generally, any plurality of such processors may be employed as appropriate to a particular application. Each of the main signal processors 105 1-105 4 includes a main analog-to-digital converter (ADC) and a corresponding output FIR filter (i.e., ADC1-ADC4 and corresponding FIR1-FIR4). The
auxiliary signal processor 120 includes a calibration ADC and four calibration output FIR filters (i.e., ADCCAL and corresponding FIRCAL1-4). - Each first input pole of the
input switch bank 110 is connected to an analoginput signal bus 106, and each second input pole of theinput switch bank 110 is connected to a rampinput signal bus 107. Similarly, each first output pole of theoutput switch bank 115 is connected to a digitaloutput signal bus 116. Each second output pole of theoutput switch bank 115, associated with the main signal processors 105 1-105 4, is connected to amismatch calculation input 117 of thetiming calibration circuit 125. Similarly, a second output pole of theoutput switch bank 115, associated with theauxiliary signal processor 120, is connected to aninterpolation input 118 of thetiming calibration circuit 125. - The
timing calibration circuit 125 includes afirst register 126 containing initial digitized data stored in a first register (1), adigital interpolation module 127, aregister array 128 containing interpolated data stored in a second register (2), a third register (3) and a fourth register (4) and amismatch calculation module 129 that provides amismatch calculation output 130 connected to thefilter coefficient switch 135. - The
background calibrator 100 provides estimation and compensation of a fixed periodic timing mismatch among the parallel array of four main signal processors 105 1-105 4. In the illustrated embodiment, estimation of timing mismatch requires theauxiliary signal processor 120 employing the additional calibration ADCCAL and the four calibration output FIR filters FIRCAL1-4. The calibration filters FIRCAL1-4 are employed to correct the timing errors. As will be shown in a subsequent discussion, the proposed calibration technique can significantly improve the signal-to-noise-and-distortion ratio (SNDR) performance for the TIADC 105. - The
background calibrator 100 employs switch positions shown inFIG. 1A for the input andoutput switch banks filter coefficient switch 135 to provide needed digital interpolation and compensation parameters. During an initial calibration phase, an input ramp signal is connected to the compensation ADCcal and corresponding filter FIRCAL1, while employing a first sampling clock clk1, wherein digitized calibration data are stored into the first register (1). Thedigital interpolation circuit 127 follows the first register (1) and employs the digitized calibration data of register (1) to estimate the next quantized data corresponding to different-phase sampling clocks (i.e., second, third and fourth sampling clocks clk2-4). The four sampling clocks clk1-4 approximately occupy zero, 90, 180 and 270 degrees of phase shift in the sampling period for the four stages of TIADC 105. - Turning momentarily to
FIG. 2 , illustrated is an embodiment of an interpolation chart, generally designated 200, constructed in accordance with the principles of the present embodiment. Theinterpolation chart 200 shows digital data samples and estimates for the calibration ADCCAL having an input ramp signal and employing the four sampling clocks clk1-4. - The interpolation operation employs an interpolation function yi,m, which may be defined as the mth sample of the calibration ADCcal with a sample clock i, (i=1-4). Then, first and second samples y1,1 and y1,2 may be seen, for the first sampling clock clk1 at sample times t1,1 and t1,2, respectively, in the
interpolation chart 200. - After employing the calibration ADCcal to collect digital data with the first sampling clock clk1, a mean function associated with the
digital interpolation module 127 may be employed to estimate the digital data associated with the calibration ADCcal using the second, third and fourth sampling clocks clk2-4. Effectively, reference data stored in register (1) is employed while sequentially using sampling clocks clk2-4 for the calibration ADCcal to sample theramp input signal 105. This action yields interpolation values y3,1 at t3,1, y2,1 at t2,1 and y4,1 at t4,1, respectively. - In general, these interpolation values may be defined between samples as shown in equations (1a)-(1c) below:
- Referring again to
FIG. 1A , the interpolated data is then stored in theregister array 128 employing the second, third and fourth registers (2)-(4). The interpolated data are regarded as ideal sampled data of the calibration ADCcal and provide sampled data for ideal clock positions of zero, 90, 180 and 270 degrees having no timing errors. As a consequence, the interpolated data in the registers (1)-(4) are used for calibration as reference data. - Then, in a next calibration phase, the calibration ADCcal samples the input ramp signal employing the second, third and fourth sampling clocks clk2-4. Output data are compared with the reference data in registers (2)-(4) and adjustments are made to the coefficients of FIRCAL2-4 employing the
mismatch calculation module 129 and themismatch calculation output 130. - The errors between actual sampled data and the reference data in registers (1)-(4) come from timing errors. In the illustrated embodiment, the timing errors may be calculated employing the
mismatch calculation module 129 according to equation (2) below.
where Δti,m is the timing error of clki (i=2-4) relative to clk1, S is the slope of the ramp signal, yi,m* is the measured digital data and yi,m is the interpolated register data. Adjustments are made to the coefficients of FIRCAL2-4 until the sample timing errors are reduced below a preselected threshold. - FIR filters typically introduce delays of many sample periods. However, FIR filters may also introduce fractional sample-period delays when sample time errors between interleaved main signal processors exist. These fractional delays may be both positive and negative depending on the filter coefficients employed. This principle is employed to compensate for timing mismatches between the main signal processors of the
TIADC 105. A more detailed explanation of employing FIR filters to compensate for timing errors may be seen in “A 10-b 120-Msample/s Time-Interleaved Analog-to-Digital Converter With Digital Background Calibration”, by Shafiq M. Jamal, et al., IEEE Journal Of Solid-State Circuits, Vol. 37, No. 12, December 2002, which is incorporated herein by reference. - Turning now to
FIG. 1B , illustrated is an alternative connection of the background calibrator ofFIG. 1A , generally designated 120, constructed in accordance with the principles of the present invention. Thebackground calibrator 120 shows theauxiliary signal processor 120 being substituted for one of the main signal processors (the secondmain signal processor 105 2, as exemplary) while it is being calibrated. In this example, theauxiliary signal processor 120 is connected between the analoginput signal bus 106 and the digitaloutput signal bus 116 while employing the second sampling clock clk2 to allow normal operation of theTIADC 105. Correspondingly, the secondmain signal processor 105 2 is connected to the rampinput signal bus 107 and themismatch calculation 117 to allow its calibration. - After minimizing the sampling time errors of the calibration ADCcal as was discussed with respect to
FIG. 1A , a final calibration is carried out for each of the main ADC1-ADC4 (i.e., main ADCi). This calibration phase again employs thetiming calibration circuit 125 as discussed with respect toFIG. 1A . Again, the relative timing mismatches of the four main ADCs relative to the references of thetiming calibration circuit 125 are reduced below a preselected threshold by adjusting the coefficients of the corresponding FIR filter (i.e., FIRi) associated with each main ADCi. Once this is accomplished, the input and output connections may again be employed as depicted inFIG. 1A . - Turning now to
FIG. 3 , illustrated is a power spectral density diagram, generally designated 300, associated with a TIADC as was discussed with respect toFIGS. 1A and 1B . The diagram 300 includes an uncalibrated output power spectral density (PSD) 305 and a calibratedoutput PSD 310. Theuncalibrated output PSD 305 showsspurious responses output PSD 310 shows the output spectrum after calibration as carried out in accordance with the principles of the present invention. - A TIADC using four time-interleaved channels of 12-bit ADCs was employed to demonstrate the benefits of compensation provided by an embodiment for periodic timing-mismatch conditions. The number of filter taps needed in the corresponding FIR filters is mainly dependent on the required ADC resolution and the amount of sample timing error encountered. For 12-bit resolution, a FIR filter having 45-taps is employed for timing error correction in each of the main ADCs. This accommodates a fixed timing mismatch error of about one percent of sampling time among the time-interleaved phase generation paths.
- The
uncalibrated output PSD 305 shows the output spectrum of the TIADC without timing calibration. The input signal frequency is fin=0.1*fs, where fs is the sampling frequency. Thespurious responses
The calibratedoutput PSD 310 shows the benefits of calibration. The magnitude of the spurious response has been suppressed by about 50 dB, which is enough suppression to provide a clear spectrum for the 12-bit TIADC. - Turning now to
FIG. 4 , illustrated is an SNDR chart, generally designated 400, showing uncalibrated and calibrated responses for a TIADC system constructed in accordance with the principles of the present invention. TheSNDR chart 400 includes anuncalibrated SNDR curve 405 and a calibratedSNDR curve 410 for normalized timing errors. For theuncalibrated SNDR curve 405, the SNDR may be seen to drop exponentially as timing error increases thereby demonstrating an SNDR of only about 30 dB at a normalized timing error of about 12 percent. Alternatively, the calibratedSNDR curve 410, showing the effect of calibration, provides an almost flat SNDR of greater than 70 dB for a normalized timing error up to about 12. - Turning now to
FIG. 5 , illustrated is a flow diagram of an embodiment of a method for calibrating a TIADC, generally designated 500, which may be used, for example, for calibrating the TIADC ofFIG. 1A orFIG. 1B . Themethod 500 is for use with a parallel array of main signal processing ADCs and corresponding FIR filters employed in a time-interleaved manner. Additionally, the method uses an auxiliary signal processor having an ADC that is also connected to a parallel plurality of output FIR filters, wherein each of these output FIR filters corresponds to one of the main output FIR filters. The auxiliary signal processor may be employed to achieve a background calibration mode of operation. - The
method 500 starts in astep 505. In astep 510, themethod 500 involves applying an input ramp signal to the auxiliary signal processor that employs a first sampling clock as a reference clock. This provides samples of the input ramp voltage that serve as reference samples for interpolation. Then, in astep 515, themethod 500 involves providing sample values that interpolate between two of the reference samples and correspond to ideal time-interleaved sampling times for the remaining number of sampling clocks employed in the TIADC. These interpolated sample values may be employed as reference data, because the ideal time-interleaved sampling times have no timing errors with respect to the first sampling clock. - The auxiliary signal processor then employs each of the remaining sampling clocks, in turn, to provide actual sample values of the input ramp signal. In a
step 520, themethod 500 involves determining a timing mismatch between the actual sample values and the interpolated sample values for each of the remaining sampling clocks. Then, in a firstdecisional step 525, themethod 500 involves further determining whether the timing mismatch, which was determined in thestep 520, is below a preselected threshold. If the timing mismatch is not below the preselected threshold, themethod 500 includes reducing the timing mismatch by modifying filter coefficients of the output FIR filter being employed with the auxiliary signal processor for the selected remaining sampling clock. Then, themethod 500 includes returning to thestep 510 and repeating the steps until the timing mismatches for all of the remaining sampling clocks are below the preselected threshold. - Upon completing of the calibration of the auxiliary signal processor employing all sampling clocks, the
method 500 involves exchanging the calibrated auxiliary signal processor for a selected main signal processor, in astep 535. The auxiliary signal processor employs the sampling clock and calibrated output FIR filter that corresponds to the sampling clock and output FIR filter of the selected main signal processor in the exchange. This allows the selected main signal processor to be calibrated in a background mode while the TIADC continues to be fully functional. - Then, in a
step 540, themethod 500 includes applying the input ramp signal to the selected main signal processor, while employing its regular sampling clock and output FIR filter, to provide an actual sample value. In astep 545, themethod 500 involves determining the timing mismatch between the actual sample value and the corresponding interpolated value. In a seconddecisional step 550, themethod 500 further involves determining whether the timing mismatch is below the preselected threshold. If it is not below the preselected threshold, the timing mismatch is reduced by modifying the filter coefficients of the selected output FIR filter. - The
method 500 involves returning to thestep 540 and executing the steps 540-550 until the remaining timing mismatch is below the preselected threshold. In response to the mismatch being below the threshold, themethod 500 involves performing a thirddecisional step 560 to determine whether the last main signal processor has been selected for calibration. If the last main signal processor has not been selected, the previously selected and now calibrated main signal processor is returned to the TIADC, and themethod 500 involves returning to thestep 535. Here, the calibrated auxiliary signal processor is exchanged for another selected main signal processor, which is to be calibrated. This sequence continues until it is determined in the thirddecisional step 560 that the last main signal processor has been calibrated, wherein themethod 500 ends in astep 565. - While the method disclosed herein has been described and shown with reference to particular steps performed in a particular order, it will be understood that these steps may be combined, subdivided, or reordered to form an equivalent method without departing from the teachings of the present invention. Accordingly, unless specifically indicated herein, the order or the grouping of the steps is not a limitation of the present invention.
- In summary, embodiments of the present invention employing a background calibrator and a method for calibrating a time-interleaved analog-to-digital converter (TIADC) have been presented. Advantages include a reduction of fixed periodic timing mismatch effects in the TIADC. The compensation utilizes FIR filters to correct the timing errors. The signal-to-noise-and-distortion ratio (SNDR) performance for a 12-bit TIADC system was shown to be significantly enhanced by the calibrator and the method.
- Although the present invention has been described in detail, those skilled in the art should understand that they can make various changes, substitutions and alterations herein without departing from the spirit and scope of the invention in its broadest form.
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