How to use cancellation drive circuit to improve common mode rejection ratio

ECG detection is the detection of weak signals under strong common-mode interference. In order to improve the common-mode rejection ratio of the circuit, a cancellation drive circuit (right leg drive) is often used to improve the common-mode rejection ratio.

The ECG signal is a differential voltage between specific points on the human body, with a signal amplitude between 0.5mV and 8mV, and a typical value of 1mV. The ECG is subject to very strong power frequency interference. Generally, the power frequency amplitude of the human body is at the V level, which is 3 orders of magnitude larger than the ECG signal. Power frequency interference often appears in the form of common mode.

It is a big challenge to detect weak ECG signals in such strong power frequency interference, which requires the op amp to have a very high common mode rejection ratio, generally between 60 and 120dB. If it is too low, it will affect the performance of the ECG machine, and if it is too high, the cost of the op amp will be unacceptable. Preamplifiers with high common mode rejection ratios, such as instrument amplifiers with a common mode rejection ratio of 80dB at 50Hz, are products with relatively good performance. In this way, the amplitude of power frequency interference still reaches one-tenth of the signal amplitude, which is unacceptable for doctors to diagnose. ECG machines generally use cancellation drive circuits to further enhance common mode rejection capabilities.

Figure 1: Cancellation-driven model

U2A and R9, R10, R11, C7 together form the right leg drive circuit, the principle of which is to take out the common mode voltage from the human body through R6, R7 and add it back to the human body. Below we use the formula to illustrate the effect of the cancellation drive, ignoring the influence of the error of C5, C6, 1A, U1B on the common mode rejection ratio in the calculation. At the same time, R10 and C7 are designed for system stability and do not affect the calculation of the common mode rejection ratio at low frequency. They are ignored in the calculation. The formula is derived as follows:

U1A and U1B act as impedance transformation followers, with:

Vcom=Vin (1)

U2A forms an inverting amplifier with a transfer function of:

Vout=-(R9*Vcom)/R8 (2)

According to Kirchhoff's current law:

(V1-Vin)* jwC11=Vin* jwC2+(Vin-Vout )/R11 (3)

The Vin transfer function is obtained as:

Vin=R8*R11*JWC2/(R8*R11* jwC1+R8*R11* jwC2+jwC1* jwC2*R8+ jwC1* jwC2*R9 ) (4)

Typical parameters of commonly used circuits:

R8=10kΩ, R11=100kΩ, R9=1M, C2=100pf. Ignoring the phase effect of the capacitor, ignoring the minimum term, the following is true:

Vin=R8*R11* jwC2/(R8+R9) (5)

Under the above parameters, the amplitude of Vsub>in is:

Vin=1/3000=-69dB (6)

The right leg drive circuit designed according to the above parameters can theoretically provide a common-mode rejection ratio of 69dB. This is a simplified cancellation drive model. In the actual circuit, considering the errors of resistance and capacitance, the influence of filter capacitors on phase, the delay of op amp, etc., the common-mode rejection capability of the right leg drive will deteriorate and even produce oscillations. The specific circuit needs to be adjusted as needed.