op amp common mode distortion–does dc bias matter?

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dfuruta

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I've been reading about common mode distortion in op amps, and I think I understand the basic idea (although not the mechanism).  I understand why it's a problem in a non-inverting configuration, and not in an inverting configuration.

I want to ask:  does this only matter for common mode AC signals, or will a common mode DC offset degrade performance?

Specifically, would one need to worry about CM distortion in cases like the attachment?

My apologies in advance if this is a foolish question!  Trying to educate myself, but I don't understand some of this very well.
 
Whoops, let me try that again.

 

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yes it matters. Reading an opamp data sheet will generally provide an input operating voltage range.

Outside that range the opamp is not expected to work properly, inside that range it can degrade when operated near the margins of that limit.

JR
 
You only worry about AC common mode swing. In both examples you show there is no AC common mode swing, just a fixed DC common mode bias. This does not (significantly) affect the opamps distortion performance.

The magnitude of the observes AC common mode distortion might vary if you superpose a DC common mode signal. However, this is not an usual situation, and the effect is likely benign.

JR talks about gross nonlinear effects when you exceed the input common mode voltage range. The result is similar to e.g. output clipping. However, as far as I understand, that's not what you're talking about.

Samuel
 
Thanks!  Good to know.

Samuel Groner said:
You only worry about AC common mode swing. In both examples you show there is no AC common mode swing, just a fixed DC common mode bias. This does not (significantly) affect the opamps distortion performance.

The magnitude of the observes AC common mode distortion might vary if you superpose a DC common mode signal. However, this is not an usual situation, and the effect is likely benign.

JR talks about gross nonlinear effects when you exceed the input common mode voltage range. The result is similar to e.g. output clipping. However, as far as I understand, that's not what you're talking about.

Samuel
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