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DC Offset at the output of OPAMP used for Differential input

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surendramalu

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Hi,

I am using a LMV321 OPAMP to amplify the difference between two audio signals, basically using it as a differential OPAMP. But I have powered up the OPAMP with a single supply. I do not have -ve supply available in my design.

When I bias the non-inverting input of the OPAMP with +Vcc/2, I get the same DC offset at the output of the OPAMP.

How do I get rid of this DC offset? decoupling caps at the OPAMP input and output do not help, nor does a high pass filter.

Thanks in Advance,
Surendra
 

You could simply capacitively couple the output (connect to the next stage with a series capacitor). When working with opamps on a single supply you usually need to reference you signals to some point other than ground. In your case you have picked VCC/2 which is fine, but the output will also be relative to VCC/2. You cannot shift it down to GND if the output would then need to be below ground.

Keith
 

surendramalu said:
Hi,

When I bias the non-inverting input of the OPAMP with +Vcc/2, I get the same DC offset at the output of the OPAMP.
Click to expand...

Yes, that's the only purpose of the input bias. Otherwise, your output cannot swing around a fixed dc operating point (in your case: Vcc/2).
Ask yourself why you have biased the input.
 

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