How to work out what minimum cmrr I need?

[throbscottle]

I'm busy trying to choose an op-amp for the input stage to my meter project - got down a few candidates!

It would help if I knew how to calculate what minimum cmrr I need though.

Input source is 10M divider, gain is 2.5 single sided non-inverting, I'm looking at amps with input bias currents of <10pA, load will be about 10K.

Hope this is enough info for someone to start helping me!

Thanks in advance

 

[audioguru]

Your amplifier is single-ended (not differential) so the CMRR of the opamp does not affect it.

 

[crutschow]

Your amplifier is single-ended (not differential) so the CMRR of the opamp does not affect it.

Actually the common-mode rejection has a theoretical effect since there is a common-mode voltage equal to the input voltage with a non-inverting amp , but the CMRR of most op amps is so high that its effect on the output can be ignored.

 

[throbscottle]

Wow, thanks! The field just got a whole lot wider!