CN104166034A - High-precision difference sampling circuit - Google Patents

Abstract

The invention belongs to the technical field of analogue integrated circuits, and particularly relates to a high-precision difference sampling circuit. The high-precision difference sampling circuit is composed of a voltage following operational amplifier, a difference sampling operational amplifier, a resistor RS1, a resistor RS2, a resistor RS3 and a resistor RS4. The forward input end of the voltage following operational amplifier is connected with the forward input end of the sampling circuit, the reverse input end of the voltage following operational amplifier is connected with the output end of the sampling circuit, the output end of the voltage following operational amplifier passes through the RS1 and the RS2 in sequence and then is connected to the ground, and the ground potential end of the voltage following operational amplifier is connected to the ground. The forward input end of the difference sampling operational amplifier is connected with the connecting point of the RS1 and the RS2, the reverse input end of the difference sampling operational amplifier passes through the RS3 and then is connected with the negative input end of the sampling circuit, and after the reverse input end of the difference sampling operational amplifier passes through the RS4, the output end of the difference sampling operational amplifier serves as the output end of the sampling circuit, and the ground potential end of the difference sampling operational amplifier is connected to the ground. The high-precision difference sampling circuit has the advantages that errors caused by voltage drop produced due to large current or remote transmission are completely eliminated, and the output precision of a power system is greatly improved. The high-precision difference sampling circuit is especially suitable for switching power supplying circuits.

Description

A kind of high-precision difference sample circuit

Technical field

The invention belongs to Analogous Integrated Electronic Circuits technical field, be specifically related to a kind of high-precision difference sample circuit.

Background technology

Difference sampling feedback circuit is a kind of the most frequently used important integrated circuit modules in Analogous Integrated Electronic Circuits design.Especially in switching power circuit, seem and be even more important, switching power circuit needs sampling and outputting voltage, and sampled signal is fed back to error amplifier, after comparing with reference voltage, by error amplifier, produce error signal and carry out loop adjustment, thereby reach the object of stable output.

And switching power circuit is as the nucleus module of consumer power supply, require its output to there is high precision, high stability and low noise feature.In switching power circuit, export exactly sample circuit with output accuracy and the closely-related module of degree of stability, conventional switch power module output sample circuit consists of sampling resistor dividing potential drop, the voltage signal that is about to sample on divider resistance, and voltage division signal error originated from input amplifier is produced to error signal.This sample circuit is simple in structure, and cost is lower, but the defect existing is also very obvious.Such as, when output load current is larger, large electric current will flow through output terminal earth potential and chip ground potential points, and this can cause two ground potential points to produce voltage difference, thereby make output voltage sampled signal inaccurate, cause switch power module output voltage precision low, adjust rate variance.

Summary of the invention

Object of the present invention, is exactly problem simple in structure for above-mentioned existing Switching Power Supply sample circuit, sampling precision is low, proposes a kind of high-precision difference sample circuit that is applicable to multiple power sources topological structure.

Technical scheme of the present invention is, as shown in Figure 1, this sample circuit is by voltage follow operational amplifier A 1, difference sampling operational amplifier A 2, and resistance R S1, RS2, RS3, RS4 form; Wherein, the positive input terminal VINP that the positive input of voltage follow operational amplifier connects sample circuit is the positive pole of sampling resistor RF2, its reverse input end and its output terminal VA1_OUT interconnection, its output terminal VA1_OUT is successively by meeting chip GND1 equivalently after RS1, RS2, its earth potential termination output equivalent ground GN2; The positive input of difference sampling operational amplifier connects the tie point of RS1 and RS2, its reverse input end meets the negative input end VINN of sample circuit after by RS3, its reverse input end meets its output terminal VA2_OUT as the output terminal of sample circuit after by RS4, and its earth potential chip termination is GND1 equivalently.The output terminal output sampled signal of sample circuit produces error signal to error amplifier.

Beneficial effect of the present invention is, the present invention utilizes the sampling principle of Differential OPAMP, directly the pressure reduction at feedback resistance two ends is carried out to fully differential sampling, thereby eliminated completely on ground wire because the error that the pressure drop that large electric current or long-distance transmissions produce brings, and the impact that in the time of can offsetting high frequency, output phase shift causes sampling, eliminate common mode high frequency noise for the impact of sampling, increased substantially output accuracy, load regulation and the line regulation of power-supply system.

Accompanying drawing explanation

Fig. 1 is traditional Switching Power Supply output sample circuit structural representation;

Fig. 2 is high-precision difference sample circuit structural representation of the present invention;

Fig. 3 is sample circuit of the present invention and chip connection scheme schematic diagram altogether;

Fig. 4 is relatively connection scheme schematic diagram of sample circuit prime of the present invention.

Embodiment

Below in conjunction with accompanying drawing, the present invention is described in detail

Traditional switch power module sample circuit, the switching power circuit of BUCK topology of take is example, conventional BUCK power supply sample circuit is as shown in Figure 1.In figure, GND1 is input and chip current potential equivalently, and GND2 is output equivalent earth potential.

By Tu Ke get, when zero load, Io=0, V _gND1=V _gND2so,,

$V_{\mathrm{sense}}=\frac{R_{F2}}{R_{F2}+R_{F1}}\×V_{\mathrm{OUT}}$

As output full load, Io=V _oUT/ Ro, so,

$V_{\mathrm{sense}}=\frac{R_{F2}}{R_{F2}+R_{F1}}\×V_{\mathrm{OUT}}+I_O\×R_{\mathrm{GND}}$

In formula, V _sensefor sampled voltage, V _oUTfor BUCK circuit output voltage, R _f1and R _f2for sampling divider resistance, Io is output load current, R _gNDfor chip earth potential between load output terminal earth potential equivalence transmission resistance.

This shows, when load current Io is larger, output sampled voltage V _senselarge while understanding than zero load, this can make output sampling inaccurate, thereby causes output voltage inaccurate, and line regulation and load regulation are low.

In order to address this problem, the present invention proposes to utilize the characteristic of operational amplifier fully differential input, the voltage at Direct Sampling feedback resistance two ends, make sampled voltage signal can not be subject to the impact of earth potential and load current, reach the effect of accurate sampling, thereby greatly improved output accuracy and linearity and the load regulation of switch power module.

As shown in Figure 2, sample circuit of the present invention is by voltage follow operational amplifier A 1, difference sampling operational amplifier A 2, and resistance R S1, RS2, RS3, RS4 form; Wherein, the positive input terminal VINP that the positive input of voltage follow operational amplifier connects sample circuit is the positive pole of sampling resistor RF2, its reverse input end and its output terminal VA1_OUT interconnection, its output terminal VA1_OUT is successively by meeting chip GND1 equivalently after RS1, RS2, its earth potential termination output equivalent ground GN2; The positive input of difference sampling operational amplifier connects the tie point of RS1 and RS2, its reverse input end meets the negative input end VINN of sample circuit after by RS3, its reverse input end meets its output terminal VA2_OUT as the output terminal of sample circuit after by RS4, and its earth potential chip termination is GND1 equivalently.The output terminal output sampled signal of sample circuit produces error signal to error amplifier

Below sampling gain of the present invention, bandwidth are made a concrete analysis of:

1. sampling gain is analyzed

Analysis chart 2 can obtain, because voltage follow operational amplifier A 1 has been connected into the form of operational amplifier, so

$\begin{array}{c}{V}_{A1\_\mathrm{OUT}}=V_{\mathrm{INP}}\\ V_{\mathrm{INP}\_A2}=\frac{R_{S2}\×V_{\mathrm{INP}}}{R_{S1}+R_{S2}}\end{array}---\left(1\right)$

V in formula _{a1_OUT}for the output of voltage follow operational amplifier A 1, V _{iNP_A2}anode input voltage for difference sampling operational amplifier A 2.V _iNPfor sample circuit positive input, R _s1with R _s2for BUCK circuit output divider resistance.

Difference sampling operational amplifier A 2 has been connected into the form of subtracter, so have:

$\frac{V_{A2\_\mathrm{OUT}}-V_{\mathrm{INP}\_A2}}{R_{S4}}=\frac{V_{\mathrm{INP}\_A2}-V_{\mathrm{INN}}}{R_{S3}}---\left(2\right)$

(1), (2) formula simultaneous can obtain:

$\frac{V_{A2\_\mathrm{OUT}}-\frac{R_{S2}\×V_{\mathrm{INP}}}{R_{S1}+R_{S2}}}{R_{S4}}=\frac{\frac{R_{S2}\×V_{\mathrm{INP}}}{R_{S1}+R_{S2}}-V_{\mathrm{INN}}}{R_{S3}}---\left(3\right)$

Get R _s1=R _s2, R _s3=R _s4, (3) formula can turn to:

$\begin{array}{c}\frac{V_{A2\_\mathrm{OUT}}-\frac{V_{\mathrm{INP}}}{2}}{R_{S4}}=\frac{\frac{V_{\mathrm{INP}}}{2}-V_{\mathrm{INN}}}{R_{S3}}\\ \⇒V_{A2\_\mathrm{OUT}}-\frac{V_{\mathrm{INP}}}{2}=\frac{V_{\mathrm{INP}}}{2}-V_{\mathrm{INN}}\\ \⇒V_{A2\_\mathrm{OUT}}=V_{\mathrm{INP}}-V_{\mathrm{INN}}=V_{\mathrm{SENSE}}-V_{\mathrm{GND}2}\end{array}---\left(4\right)$

V wherein _gND2for system output equivalent earth potential.By (4) Shi Ke get, sample circuit output VA2_OUT accurately equals the pressure reduction at sampling resistor RF2 two ends, and is not subject to the impact of output load current.

2. bandwidth analysis

By Fig. 2 analysis, can be obtained, the bandwidth of sample circuit is mainly by two operational amplifier A 1, and the decision of the closed-loop bandwidth of A2, establishes amplifier A1, and A2 is that first order pole is stablized amplifier, and the open-loop gain of establishing amplifier A1 is A _a1, open loop dominant pole is P _a1,

The open-loop gain of amplifier A2 is A _a2, open loop dominant pole is P _a2.

Can obtain, the closed-loop bandwidth of amplifier A1 part is:

BW _A1＝P _A1(1+LG)＝P _A1(1+A _A1*β ₁)＝P _A1(1+A _A1)＝A _A1*P _A1 (5)

In formula, P _a1for the open loop dominant pole of amplifier A1, the loop gain that LG is A1, β ₁for the feedback factor of A1, A _a1open-loop gain for amplifier A1.

The closed-loop bandwidth of amplifier A2 part is:

In formula, P _a2for the open loop dominant pole of amplifier A2, the loop gain that LG is A2, β ₂for the feedback factor of A2, A _a2open-loop gain for amplifier A2.Because the gain A of operational amplifier _a1and A _a2very large, therefore two amplifiers have very large closed-loop bandwidth, can guarantee the sampling precision under high frequency, the high frequency small-signal of feedback output end shake in time.

3. the inhibition of high frequency common mode noise

As everyone knows, can be because the action of switching tube turn-on and turn-off produce high frequency common mode noise on output terminal and earth potential in switching power source chip, this high frequency common mode noise also can be reflected in output sampling resistor (as R in Fig. 2 _f2) two ends, by above analysis, being easy to get, the sample mode of Fig. 1 can not suppress this high frequency common mode output noise, and the sampling with high precision resistance that the present invention proposes can suppress this high frequency common mode noise effectively.The further raising sample circuit of consideration based on to(for) high frequency common mode noise rejections such as burr spikes, the present invention is just like Fig. 3, two kinds of connections of Fig. 4, wherein R _s1=R _s2, R _s3=R _s4.Below will analyze contrast to two kinds of connections.

As shown in Figure 3, the earth potential of operational amplifier A 1 and A2 is all connected in chip ground potential GND 1, as feedback resistance R _f2on two ends, produce high frequency common mode noise v _nwhen (j ω), $\left\{\begin{array}{c}{V}_{\mathrm{INN}}\left(\mathrm{j\ω}\right)=V_{\mathrm{GND}2}+v_n\left(\mathrm{j\ω}\right)\\ V_{\mathrm{INP}}\left(\mathrm{j\ω}\right)=V_R+V_{\mathrm{GND}2}+v_n\left(\mathrm{j\ω}\right)\end{array}\right.,$ Wherein, V _rdirect current pressure drop for sampling resistor two ends.

Can obtain:

$\begin{array}{c}{V}_{A1\_\mathrm{OUT}}\left(\mathrm{j\ω}\right)=\frac{[V_R+V_{\mathrm{GND}2}+v_n\left(\mathrm{j\ω}\right)]\frac{A_{A1}}{1+\mathrm{j\ω}/p_{A1}}}{1+\frac{A_{A1}}{1+\mathrm{j\ω}/p_{A1}}}=\frac{[V_R+V_{\mathrm{GND}2}+v_n\left(\mathrm{j\ω}\right)]\×A_{A1}}{1+\mathrm{j\ω}/p_{A1}+A_{A1}}\\ ._{.}^{.}{V}_{A2\_\mathrm{OUT}}=V_{A1\_\mathrm{OUT}}\left(\mathrm{j\ω}\right)-V_{\mathrm{INN}}\left(\mathrm{j\ω}\right)=\frac{[V_R+V_{\mathrm{GND}2}+v_n\left(\mathrm{j\ω}\right)]\×A_{A1}}{1+\mathrm{j\ω}/p_{A1}+A_{A1}}-[V_{\mathrm{GND}2}+v_n\left(\mathrm{j\ω}\right)]\end{array}---\left(7\right)$

By (7) Shi Ke get, work as v _n(j ω) frequency is higher while even surpassing the closed-loop bandwidth of amplifier A1, the output V of A1 _{a1_OUT}can produce phase shift, signal V _{a2_OUT}(j ω) is through R _s1and R _s2after dividing potential drop with V _iNN(s) signal asks poor, the output V of sample circuit _{a2_OUT}≠ V _r, can produce larger sampling error.

As shown in Figure 4, when the earth potential of operational amplifier A 1 is connected on the negative terminal VINN of sample circuit, on feedback resistance RF2 two ends, produce high frequency common mode noise v _nwhen (j ω), can obtain:

$\begin{array}{c}{V}_{A1\_\mathrm{OUT}}\left(\mathrm{j\ω}\right)=[V_{\mathrm{INP}}\left(\mathrm{j\ω}\right)-V_{\mathrm{INN}}\left(\mathrm{j\ω}\right)]\×\frac{\frac{A_{A1}}{1+\mathrm{j\ω}/p_{A1}}}{1+\frac{A_{A1}}{1+\mathrm{j\ω}/p_{A1}}}+V_{\mathrm{INN}}\left(\mathrm{j\ω}\right)\\ =[V_R+V_{\mathrm{GND}2}+v_n\left(\mathrm{j\ω}\right)-V_{\mathrm{GND}2}-v_n\left(\mathrm{j\ω}\right)]\×\frac{\frac{A_{A1}}{1+\mathrm{j\ω}/p_{A1}}}{1+\frac{A_{A1}}{1+\mathrm{j\ω}/p_{A1}}}+V_{\mathrm{GND}2}+v_n\left(\mathrm{j\ω}\right)\\ \≈[V_R+V_{\mathrm{GND}2}+v_n\left(\mathrm{j\ω}\right)\\ \·_{\·}^{\·}{V}_{A2\_\mathrm{OUT}}=V_{A1\_\mathrm{OUT}}\left(\mathrm{j\ω}\right)-V_{\mathrm{INN}}\left(\mathrm{j\ω}\right)=V_R\end{array}---\left(8\right)$

Derivation by (8) formula can obtain, and the connection of Fig. 4 can effectively suppress the impact of high frequency common mode noise, improves the common-mode rejection ratio of sampling module.

Comprehensive above analysis can obtain, and as shown in Figure 2, the circuit diagram of sampling with high precision circuit as shown in Figure 4 for application scheme of the present invention.Sample circuit of the present invention has three features: 1. sampling precision is high, the signal at direct differential sampling feedback resistance two ends, and be not subject to the impact of the signals such as earth potential; 2. sample strip is roomy, and the two stage amplifer connected mode of sample circuit has very large bandwidth, guarantees the sample circuit high-frequency signal of can accurately sampling; 3. common mode inhibition capacity is strong, adopts relatively earthy thought, and the impact that while offsetting high frequency, output phase shift causes sampling is eliminated common mode high frequency noise for the impact of sampling.

Claims (1)

1. a high-precision difference sample circuit, is characterized in that, this sample circuit is by voltage follow operational amplifier, difference sampling operational amplifier, and resistance R S1, RS2, RS3, RS4 form; Wherein, the positive input of voltage follow operational amplifier connects the positive input terminal of sample circuit, its reverse input end and the interconnection of its output terminal, and its output terminal is successively by ground connection after RS1, RS2, its earth potential end ground connection; The positive input of difference sampling operational amplifier connects the tie point of RS1 and RS2, and its reverse input end connects the negative input end of sample circuit after by RS3, and its reverse input end connects its output terminal as the output terminal of sample circuit, its earth potential end ground connection after by RS4.