AnsweredAssumed Answered

Electret mic with AD620 IN amp - output hitting positive rail.

Question asked by Dave1989 on Aug 7, 2013
Latest reply on Aug 15, 2013 by KJBob

Hey everyone,

 

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:

AD620 preamp.JPG

 

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:

 

AD620 preamp.JPG

 

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,

 

David.

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