Three Port optical Isolation amplifier

I agree as indicated in #10,#11

alec_t said:

The gain of the AD620 is set at ~2000 by the 47 Ohm resistor. If you have ~6VDC at its output that is because the CA3140 has an input offset voltage of ~6/2000 = 3mV.

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Thanks. But with 47 ohms, the gain of the AD620ANZ is 1052 approximately. From the data sheet

Thanks gentle individuals for your contributions. I am very grateful!

I have another question and this time it is about the cascaded system (V-F converter + optocoupler + F-V converter + a noninverting amplifier - gain of 2, + a lowpass filter).

This is what I had when I applied dc offset to my system so as to check its linearity:



However, by the time I include AC signal, I got this kind of response:



The above frequency response should resemble a low pass filter. I do not know why I am having such response. Thanks in advance for your feedback

This the circuit diagram for the above response:


Thanks