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CMRR - how to calculate?
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Mark:
On the face of it, this should be a simple answer, but a recent enquiry from a customer made me think twice.   


The customer has requested "CMRR >100dB (bandwidth)".


I am not in direct contact with the customer so I can't ask directly, but, what do you think this means? 

I have a 16bit ADC system with 300VRMS input, digital output, 3kV AC isolation barrier. 

The method I have used to calculate CMRR (or CMR) in the past has been to apply a DM signal of, say, 150VRMS and observe and record 150VRMS (Vdm) from the digital output. 
Then, short the inputs and apply a CM input of 150VRMS between those shorted inputs and the protective earth, recording the voltage at the output of the ADC as the observed Vcm. 
CMRR in dB is then = 20 log (Vdm/Vcm). 


Given our 16 bit system, doesn't that mean that we need to be seeing zero output when the common mode signal is applied?  Otherwise a change in the LSB of our 16 bit system would result in a CMRR of 96dB? 

Any thoughts would be appreciated.  Maybe I'm calculating this wrongly or misunderstanding the customer's requirement.   Maybe the customer doesn't know what they are asking for (quite often the requirement specification is based on a competitor's product that they are trying to tie into a contract).  I would really like to get to the bottom of this! 

Thank you for your help! 
Mark. 

Marco:
Maybe he just wants some safety margin? Especially if it uses dividers it will drift ... to the point where I think the spec will be hard to reach, this is more in the domain of a direct differential to single ended amplifier.

Why not just measure the CMRR of the analogue front end over the required measurement bandwidth? (Also for various temperatures if you want to be complete.) You could put on a jumper to reduce the attenuation a bit just for the CM measurement and still use the ADC output.

PS. oh, I guess the whole thing floats so it doesn't need a differential amplifier at all.
Mechatrommer:

--- Quote from: Mark on May 21, 2020, 02:18:19 pm ---The customer has requested "CMRR >100dB (bandwidth)".

--- End quote ---
at what bandwidth? CMRR is different at different bandwidth.


--- Quote from: Mark on May 21, 2020, 02:18:19 pm ---The method I have used to calculate CMRR (or CMR) in the past has been to apply a DM signal of, say, 150VRMS and observe and record 150VRMS (Vdm) from the digital output. 
Then, short the inputs and apply a CM input of 150VRMS between those shorted inputs and the protective earth, recording the voltage at the output of the ADC as the observed Vcm. 
CMRR in dB is then = 20 log (Vdm/Vcm). 

--- End quote ---
without looking at the detail of your circuit, i assume it is correct.


--- Quote from: Mark on May 21, 2020, 02:18:19 pm ---Given our 16 bit system, doesn't that mean that we need to be seeing zero output when the common mode signal is applied?  Otherwise a change in the LSB of our 16 bit system would result in a CMRR of 96dB? 

--- End quote ---
correct
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