# Maximum common mode voltages

1. Jun 12, 2016

### Ianlamb

• Member advised to use the homework template for posts in the homework sections of PF.
Hi,
I have this question.

An instrumentation measuring system uses a differential amplifier having a CMRR of 120 dB and differential gain of 105. The maximum differential input signal is 60 μV. If the amplified noise voltage is specified to be not more than 1% of the maximum output voltage, calculate the maximum common mode voltage that can be present in the input to the amplifier.

Just need to know if this is correct or I am on the right track.

I have the formula

CMRR=20log10(Adiff/Acm)

Then

120=20log10(10^5/Acm)

Then

120/20=log10(10^5/Acm)

Then

6=log10(10^5/Acm)

Then taking anti logs

1000000=10^5/Acm

Transpose this into

Acm=10^5/1000000=0.1volt

Is this correct? as when I plug the 0.1 back into the original equation I do not get the original 120dB answer.

Thanks.

2. Jun 12, 2016

### Tom.G

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