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Help me solve my DC offset

post #1 of 7
10/30/10 at 4:44pm
Thread Starter 

I have this amp I made this week and I have a little problem with it...but now it's fixed.


Edited by Avro_Arrow - 10/30/10 at 6:48pm
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post #2 of 7
10/30/10 at 5:13pm

What type is IC1 and IC2? Possible offset due to input bias currents. Temporarily jumper out (short-circuit) C1 and C2 to check for this condition.

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post #3 of 7
10/30/10 at 5:35pm
Thread Starter 

IC1/IC2 are LT1363CN8.

 

I'll short out C1/C2 and see if the offset goes away.

I think I measured 20 mV at the +in on the op amp.

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post #4 of 7
10/30/10 at 5:54pm
Thread Starter 

That seemed to be it. Thanks

 

I guess I will have to low the value for R2/R7 and raise the value for C1/C2

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post #5 of 7
11/1/10 at 8:31am

?

 

Why not leave the OP up so others can learn too?

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post #6 of 7
11/1/10 at 8:50am
Thread Starter 

Ok, but just because you asked nicely...

 

Here is the schematic in question.

Maybe someone better at math than me can run the

number and figure out my mistake.

 

The Op-amps are LT1363CN8.

 

Problem Child 3.png
 

Quote:
Originally Posted by nikongod View Post

?

 

Why not leave the OP up so others can learn too?

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post #7 of 7
11/1/10 at 9:08am
Thread Starter 

The problem was that the LT1363 was drawing it's input bias current through the 200k

input resistor creating approximately 30 mV DC offset which was then amplified through

the circuit. There was an unacceptable 150 mV DC offset at the output of the OPA633 buffers.

Removing the DC blocking caps was the temporary fix. Re-designing the input circuit

will be required to provide both the proper input bias and DC blocking.

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