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?
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.