I've come across this circuit diagram in Biomedical Digital Signal Processing by Willis J Tompkins and in the description it says it is to do with ECG amplification. The input to the non-inverting terminal comes from the output of an instrumentation amplifier. But I am not exactly sure what is happening in this part of the circuit. Would anyone be able to give a brief overview of what is happening or point me in a general direction of where to look further?

enter image description hereThanks

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    \$\begingroup\$ Can you be more specific than "what is happening"? It's not very clear what you are asking. \$\endgroup\$ – Elliot Alderson Oct 15 '18 at 14:52
  • \$\begingroup\$ The 3.3Meg and 1uF produces a 3 second timeconstant in a high pass filter; the 3dB corner is 2*pi slower, at about 0.05 cycles per second. Very slow, but does let you the chemical recovery actions of the brain. In other words, sinusoids of period 20 seconds will be attenuated by 3dB (1/1.414) compared to 5 second or 2 second or 0.1 second or 0.001 second waveforms. \$\endgroup\$ – analogsystemsrf Oct 15 '18 at 17:51

This circuit is a non inverting amplifier. It amplifies a signal applied to the left of the 1 uF capacitor. The "Reset" switch is a bit odd, it might be that due to the presence of the 1 uF capacitor that there is a long time constant which might allow the DC level to drift off out of the range the amplifier can handle. Then a reset might force it back into "view".

The 4.7 k ohm, 150 kohm and 10 nF capacitor determine the gain which will be around 32 (150k/4.7k) for low frequencies and 1 (one) for high frequencies. The cutoff frequency is around 100 Hz (1/(150k * 10 nF).

The 3.3 Mohm resistors are a bit excessive in their value. The one between + input and ground sets the DC voltage of the + input to 0 V. The 3.3 Mohm resistor at the - input is there to decrease the influence of the DC current flowing into the opamp's inputs. As both inputs have a 3.3 M ohm series resistor, both inputs will see the same voltage due to the opamp's input current.

If you have not read Opamps for Everyone yet then I suggest that you do as these kinds of circuits are explained in that free Ebook.

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  • \$\begingroup\$ Yes, the switch is normally a FET that is triggered when the output saturates too high or too low (e.g. power on, adjustment of the electrodes). The ECG signal has clinically-significant features down to 0.05 Hz, which requires a long time constant and the lower 3.3M resistor. Physicians are not willing to wait that long when the circuit saturates, hence the automatic reset. The upper 3.3M resistor is to eliminate a voltage offset that would otherwise occur due to the op-amp's bias current. \$\endgroup\$ – DrSheldon Oct 15 '18 at 18:20

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