Hello,

Overview: I have implemented the forward path of this circuit as shown here except ONE change: Instead of using 6k|| 60k, I have used two 2.7k resistors (G=10). The total gain of the circuit is supposed to be 1010 or 1V/1mV over the entire range of 0.3 Hz to 1 kHz. I have a Rigol DG4062 function generator which generates EMG signal. I used that as input signal to the circuit.

Issue:

When I measure the output signal amplitude, the signal is substantially lower than expected. The gain is 320 to 500 for frequencies >200 Hz. As the frequency reduces, the gain tends towards 1k.

I measured the individual gain of each stage.

The InAmp Stage provides a gain of 6-10 in the freq range 1 Hz to 1 kHz.

The BPF stage shows a gain of 65- 100 range.

If I measure the combined gain of both stages, I am unable to achieve the highest desired gain.

I thought the feedback path is not necessary for benchtop testing. Is that wrong?

Could anyone please help me understand why there is a gain reduction during the implementation of the circuit?

Thank you.