- proposed bandwidth of amplifier ?
- output impedance of input source (IN1-IN2)
- input cable length
- Is the gain of 1000 000 really needed for this circuit
Related to schematics :
- input offset voltage temperature coefficient is 18 uV/C , for gain 1000000 it means 18 V per degree - so this opamp wont work stable for high gains
- it is better to adjust offset by offset adjustment terminals of opamp N1 and N2 , and connect noninverting opamps inputs directly to ground
- the gain adjustment is done poorly and is not immune to external electromagnetic field if pcb was not properly routed and shielded
- try to use differential mode to connect to signal source - for long cables this will reduce 50 Hz noise .
- C1 to C4 need to be increased , the order of increase must be decided by max gain and measurement . I can try to estimate but leave it to you .
- circuit bandwidth must be reduced as much as possible .
- if that above wont help 50 Hz notch filter must be used after first amplifier.
- low noise instrumental opamps is better choise , Check out also opamps with chopper stabilization for noise and bandwidth related requirements .
- search internet for ECG circuits - those are designed for high gain and exposed to outside electromagnetic field. Look also to Analog's app notes and seminars - it is good source to find answers for your problem