I'm trying to amplify a signal from an electret mic and feed the amplified signal into an ADC . The bandwidth needed for the application is 1Hz - 5000Hz.
The following circuit diagram was suggested by Robert Blowsky:
I didn't want to start my measurements with a mic, as I prefered to check the circuit with a signal generator first.
So I ommited the mic and the biasing resistors and connected the following circuit:
At the output I see a large DC offset - about the size of +9V ( the positive rail voltage ).
As I change the positive rail voltage, the output follows ( regardless of input signal amplitude or frequency ).
The signal generates a pure sinusoid with no DC offset, and an amplitude of 20mV pp.
I measured the DC voltages at all the nodes and they are fine:
There isn't a significant DC offset voltage at the input to the AD620:
The voltage at the -IN input is 0.002V and at the +IN it is the same ( measured ).
The voltage at the REF pin is 0V ( measured ).
The output is hitting the rail regardless of the gain resistor I put between legs 1 and 8.
I checked, double checked and triple checked all the voltages everywhere on the circuit and the input to the AD620 is exactly the voltage the signal generator generates.
I tried to replace the IC - I have 5 of them and they all do the same thing.
Thank you for your patience,