What is the principle of the right leg drive circuit? It is necessary for ECG acquisition and has simulation files!

The figure below is a typical ECG amplifier circuit. AD620 is a 3-op-amp instrumentation amplifier, and AD705J constitutes the right leg drive circuit. The main function of the right leg drive circuit is to improve the common-mode rejection ratio and reduce 50Hz or 60Hz power frequency interference.

So what is the principle of the right leg drive circuit?

The right leg drive circuit can be simplified into the following block diagram, Vi_cm is the input common-mode voltage, Vf_cm is the feedback voltage, Vsum_cm is the sum of the input and feedback voltage, Vo_cm is the output voltage, the pre-instrumentation amplifier has an amplification factor of A=1 for the common-mode signal, and the amplification factor of the right leg drive circuit is about several dozen times, which is a reverse proportional amplification.

According to the above figure, we can get the input and output relationship of the system:

Vo_cm = Vsum_cm * A = Vsum_cm

Vsum_cm = Vi_cm + Vf_cm

Vf_cm = -F*Vo_cm

Combining the above two formulas we get:

Vo_cm = Vi_cm/ (1 + F)

The amplification factor F is relatively large, so the input common-mode voltage is attenuated, Vo_cm will be very small, and the common-mode rejection ratio is the ratio of the differential-mode gain to the common-mode gain, so the common-mode rejection ratio can be improved.

In summary, the common-mode voltage is extracted, amplified in reverse, and then fed back to the human body. The common-mode signal of the human body will be suppressed after being superimposed on the reverse-amplified common-mode signal.

The following figure is a simulation circuit diagram of the right leg drive based on multisim. ( How to obtain the simulation file: WeChat public account: Engineer watching the sea Backstage reply: right leg drive ), U1A is used as an adder to replace the pre-instrumentation amplifier (common mode gain is 1), and its output Vo_cm = Vi_cm + Vf_cm, U1B is the right leg drive part, which is a reverse proportional amplification with a magnification of -40 times.

The simulation waveform is shown in the figure below. The two light blue curves are the output and summed waveforms Vo_cm and Vsum_cm, respectively. It can be seen that the amplitudes of the two are very low, close to the 0 level, which means that the common mode voltage superimposed on the human body is effectively reduced (close to GND, which is another interesting content, and GND-related content will be introduced later), and the input common mode signal is greatly suppressed, from 100mV to 2.5mV. According to the previous analysis, theoretically it should be reduced to: Vo_cm = Vi_cm/ (1 + F) = 100/41 = 2.439 mV, and the simulation is consistent with the theoretical calculation results.

It is worth mentioning that common-mode signals can only be suppressed but not completely removed. At the same time, since the circuit cannot be completely symmetrical, part of the common-mode signal will still be converted into differential-mode signals and amplified, and eventually appear on the spectrum. Therefore, it is necessary to add hardware or software measures to further suppress common-mode interference (50Hz power frequency interference).

The above is the discussion about the right leg driving circuit. Have you learned enough?