A high common-mode rejection ratio physiological electrical signal amplification circuit

Abstract

The present invention proposes a high common-mode rejection ratio physiological electrical signal amplification circuit, which can provide an extremely high common-mode rejection ratio while suppressing DC interference, and meet the demand for high-precision measurement of physiological electrical signals. The invention is characterized in that it includes: a pre-stage amplifier circuit and an interference suppression and output circuit, the pre-stage amplifier circuit includes amplifiers A1 and A2, the interference suppression and output circuit includes amplifiers A3 and A4, resistors R7, R8, R9 and R10, and capacitors C1 and C2; wherein the amplifiers A1, A2 and A4 are instrumentation amplifiers; the IN-end of the amplifier A1 is connected to the IN+ end of the amplifier A2 and obtains an input signal IN-, the IN+ end of the amplifier A1 is connected to the IN-end of the amplifier A2 and obtains an input signal IN+; the output end of the amplifier A1 is connected to the IN-end of the amplifier A4 via the capacitor C1, and the output end of the amplifier A2 is connected to the IN+ end of the amplifier A4 via the capacitor C2.

Description

Technical Field

[0001] The invention relates to a physiological electrical signal amplifying circuit with a high common mode rejection ratio, which is applied to a pre-signal amplification of a physiological electrical measuring instrument. Background Art

[0002] The physiological electrical signal preamplifier is an important component of the physiological electrical measuring instrument (i.e., electrocardiograph). Its function is to amplify weak signals with high fidelity for further processing, recording or display. The physiological electrical preamplifier generally adopts a two-electrode differential amplifier that is symmetrical to the ground. The measured physiological electrical signal adopts a differential input mode and becomes a differential mode signal. The interference signal, especially the power frequency interference that falls within the spectrum range of most bioelectric signals, is mainly a common mode signal of equal magnitude and polarity to the input end of the differential amplifier. Therefore, in the process of recording physiological electrical signals, the preamplifier is required to have a high ability to resist common mode interference.

[0003] The three-op-amp instrument amplifier is the most commonly used physiological electrophysiological preamplifier (see Figure 1 ), it can not only provide very high input impedance, but also effectively suppress common mode noise such as power frequency interference. However, in physiological electrical measurement, due to the different placement of electrodes on the human body surface, the electrolyte between the electrodes and the skin is also different, resulting in a DC voltage of more than several millivolts at the input of the amplifier. In addition, there is a certain potential difference between various parts of the human body surface. There will always be a DC signal dozens of times larger than the useful signal at the input of the signal detection amplifier, which limits the gain of the first stage of the "three op amps". This limits the further improvement of the common mode rejection ratio. This DC signal will also have an impact effect on the RC coupled high-pass filter (R2, C1), which is called DC interference, also called baseline drift. Therefore, in order to reduce DC interference, patients are generally required to lie flat on the bed and remain calm to obtain better ECG signals. Summary of the invention

[0004] In order to overcome the problems existing in the prior art, the present invention proposes a high common mode rejection ratio physiological electrical signal amplification circuit, which can provide an extremely high common mode rejection ratio while suppressing DC interference, and meet the demand for high-precision measurement of physiological electrical signals. The specific technical contents are as follows:

[0005] A high common mode rejection ratio physiological electrical signal amplification circuit, comprising: a pre-stage amplification circuit and an interference suppression and output circuit, wherein the pre-stage amplification circuit comprises amplifiers A1 and A2, and the interference suppression and output circuit comprises amplifiers A3 and A4, resistors R7, R8, R9 and R10, and capacitors C1 and C2; wherein the amplifiers A1, A2 and A4 are instrument amplifiers;

[0006] The IN- terminal of the amplifier A1 is connected to the IN+ terminal of the amplifier A2 and obtains the input signal IN-, and the IN+ terminal of the amplifier A1 is connected to the IN- terminal of the amplifier A2 and obtains the input signal IN+; the output terminal of the amplifier A1 is connected to the IN- terminal of the amplifier A4 via the capacitor C1, and the output terminal of the amplifier A2 is connected to the IN+ terminal of the amplifier A4 via the capacitor C2;

[0007] Resistors R7 and R8 are connected in series between the output end of the amplifier A1 and the output end of the amplifier A2, and resistors R9 and R10 are connected in series between the IN+ end and the IN- end of the amplifier A4; the connection point of the resistors R7 and R8 is connected to the IN+ end of the amplifier A3, the connection point of the resistors R9 and R10 is connected to the output end of the amplifier A3, and the IN- end of the amplifier A3 is feedback-connected to the output end.

[0008] The beneficial effects of the present invention are: a preamplifier circuit with double-ended differential input and double-ended differential output is adopted on the basis of an instrument amplifier, the structure is simple and easy to debug, and a guarantee is provided for the suppression of DC interference in the later stage, and a very high common mode rejection ratio is provided under the condition of suppressing DC interference, which is particularly suitable for high-precision measurement of physiological electrical signals. According to actual measured applications, patients do not need to lie flat on the bed, and can even have a small amount of exercise without affecting the measurement of ECG signals. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Figure 1 The circuit diagram of the physiological electrical preamplifier in the prior art.

[0010] Figure 2 The circuit diagram of the high common mode rejection ratio physiological electrical signal amplifying circuit of the present invention is shown in FIG.

[0011] Figure 3 This is a normal ECG signal waveform.

[0012] Figure 4 It is a waveform diagram of the electrocardiogram signal amplified by the present invention. DETAILED DESCRIPTION

[0013] Combine the following Figures 2 to 4 , further describe this application scheme:

[0014] See attached Figure 2, a high common mode rejection ratio physiological electrical signal amplification circuit, comprising: a pre-stage amplifier circuit and an interference suppression and output circuit, the pre-stage amplifier circuit comprising amplifiers A1 and A2, the interference suppression and output circuit comprising amplifiers A3 and A4, resistors R7, R8, R9 and R10, and capacitors C1 and C2; wherein the amplifiers A1, A2 and A4 are instrumentation amplifiers; the IN- terminal of the amplifier A1 is connected to the IN+ terminal of the amplifier A2 and obtains an input signal IN-, the IN+ terminal of the amplifier A1 is connected to the IN- terminal of the amplifier A2 and obtains an input signal IN+; The output end of the amplifier A1 is connected to the IN- end of the amplifier A4 via the capacitor C1, and the output end of the amplifier A2 is connected to the IN+ end of the amplifier A4 via the capacitor C2; resistors R7 and R8 are connected in series between the output end of the amplifier A1 and the output end of the amplifier A2, and resistors R9 and R10 are connected in series between the IN+ end and the IN- end of the amplifier A4; the connection point of the resistors R7 and R8 is connected to the IN+ end of the amplifier A3, the connection point of the resistors R9 and R10 is connected to the output end of the amplifier A3, and the IN- end of the amplifier A3 is feedback-connected to the output end.

[0015] Circuit principle:

[0016] 1) Amplifiers A1 and A2 form a preamplifier circuit with double-ended differential input and double-ended differential output. Amplifiers A1 and A2 use high-precision instrumentation amplifiers, and amplifiers A1 and A2 are strictly debugged, setting their gain = 5, and the gain difference <0.02%, and the common mode rejection ratio (CMRR) = 90dB.

[0017] The voltage of the input signal IN+ is: ViO+Vi1; the voltage of the input signal IN- is: ViO+Vi2; wherein ViO is the common mode component, and Vi1 and Vi2 are the differential mode components; thus, it can be concluded that:

[0018] Vo1 = -5*(Vi2-Vi1);

[0019] Vo2=5*(Vi2-Vi1);

[0020] After 5 times amplification, it is still an accurate double-ended differential output, and its common-mode component has been suppressed (CMRR: 90dB); and the 5 times gain is to reduce the impact of the DC signal on the subsequent circuit.

[0021] 2) The interference suppression and output circuit (network) is composed of amplifiers A3 and A4, resistors R7, R8, R9 and R10, and capacitors C1 and C2, where Voi1 and Voi2 are the differential mode components of the output voltages Vo1 and Vo2 respectively, the capacitors C1 and C2 are 10uF, the resistors R7 and R8 are 10KΩ, and the resistors R9 and R10 are 1MΩ.

[0022] The input signal of amplifier A3 is taken from the midpoint voltage Vo5 of the two series resistors R8 and R9 between the output terminals of amplifiers A1 and A2, that is: Vo5 = (Vo1 + Vo2) / 2;

[0023] When only the differential-mode signal is output Vo1=-Vo2, Vo5=0, so the output voltage Vo6 of amplifier A3=0, which is equivalent to grounding; and when both common-mode voltage and differential-mode signal are input, the total output of amplifier A3 only includes the common-mode part of the input signal, that is, Vo6=(Voi1+Voi2) / 2; thus, the common-mode signal is directly added to the input end of amplifier A4 without the voltage divider of the RC coupling circuit, avoiding the reduction of the overall common-mode rejection ratio of the circuit due to the mismatch of the RC coupling circuit.

[0024] Amplifier A4 is an instrumentation amplifier. When a common-mode DC signal directly enters amplifier A4, it will not have an impact on the RC-coupled high-pass filter, effectively suppressing DC interference (baseline drift).

[0025] The differential output of the circuit can be calculated as follows:

[0026] Vo7=(Vo4–Vo3)*d4=10*(Vi2-Vi1)*d4;

[0027] Where: d4 is the differential mode gain of the integrated instrument amplifier.

[0028] The high-pass cutoff frequency fc of the circuit can be expressed as:

[0029] Among them, capacitor C1 is a tantalum electrolytic capacitor with low leakage, and its capacitance is 10uF; resistor R9 is a metal film resistor, and its resistance is 1MΩ, so the high-pass cut-off frequency obtained is 0.016Hz.

[0030] The common mode rejection ratio CMRR T of the entire circuit can be calculated by the following formula:

[0031] 1CMRR T≈1CMRR F+1CMRR S, resulting in 1CMRR T>160dB; where CMRR F and CMRR S are the common-mode rejection ratios of the first and second stages of the amplifier, respectively.

[0032] See attached Figure 3 and 4By comparing the waveforms of normal ECG signals and ECG signals amplified by the present invention, it can be seen that the waveforms are obviously distorted due to baseline drift, and the baseline drift is well suppressed, retaining important information of the ECG signal.

[0033] The above-mentioned preferred implementation modes should be regarded as illustrative examples of the implementation modes of the present application scheme. Any technical deductions, replacements, improvements, etc. that are identical or similar to the present application scheme or made based on this scheme should be regarded as within the scope of protection of this patent.

Claims

1. A high common mode rejection ratio physiological electrical signal amplification circuit, characterized in that: include: A pre-amplifier circuit and an interference suppression and output circuit, wherein the pre-amplifier circuit includes amplifiers A1 and A2, and the interference suppression and output circuit includes amplifiers A3 and A4, resistors R7, R8, R9 and R10, and capacitors C1 and C2; wherein the amplifiers A1, A2 and A4 are instrument amplifiers; The IN- terminal of the amplifier A1 is connected to the IN+ terminal of the amplifier A2 and obtains the input signal IN-, and the IN+ terminal of the amplifier A1 is connected to the IN- terminal of the amplifier A2 and obtains the input signal IN+; the output terminal of the amplifier A1 is connected to the IN- terminal of the amplifier A4 via the capacitor C1, and the output terminal of the amplifier A2 is connected to the IN+ terminal of the amplifier A4 via the capacitor C2; Resistors R7 and R8 are connected in series between the output end of the amplifier A1 and the output end of the amplifier A2, and resistors R9 and R10 are connected in series between the IN+ end and the IN- end of the amplifier A4; the connection point of the resistors R7 and R8 is connected to the IN+ end of the amplifier A3, the connection point of the resistors R9 and R10 is connected to the output end of the amplifier A3, and the IN- end of the amplifier A3 is feedback-connected to the output end; The gains of the amplifiers A1 and A2 are 5 respectively, and the common mode rejection ratio is 90dB; The capacitor C1 is a tantalum electrolytic capacitor with a capacitance of 10uF; the resistor R9 is a metal film resistor with a resistance of 1MΩ.

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