We are using the AD8421 in our product and seeing a test failure where the CMRR test we are doing is failing.

For example:

1.) On a good channel the AD8421 is the front-end of our signal path and when we doing our common mode rejection ratio test with our PCBA test fixture and we get a value of about 0.009 mVpp out of our system. This is a pass for our testing.

2.) On another channel there is a different channel being measured with the same circuit copied over and the measured value out of our system is around 0.200 mVpp that is way outside of our specification (failure).

3.) A copy of the first stage of our signal path is shown below.  Comparing a good channel with this bad channel we believe we have identified the problem being related to the input of U104 but haven't been able to pin point the underlying issue yet.  When we short the input of the amplifier together (short across the not populated part C237) the measured value out of our system is then 0.009 mVpp. This seems to indicate to us that the problem is not in the rest of the signal path but with something on the front-end of the AD8421 (U104). Thoughts?

4.) Reading the datasheet on page 23 there is a discussion of RFI and components that will impact the CMRR of the amplifier.  Some of these parts that we have not installed on our board, but we have the footprint available.  Maybe these should be installed? Recommended values? Comments?


5.) Working from the outside of our product to the inside to the amplifier we have a  connector to the outside world, this then routes to some headers that are jumpered together and this then routes to the input shown in the schematic.  Not sure what I could be checking on that would effect the CMRR because the connections look good.  Maybe extra stray capacitance? Thanks for any comments.

Thanks for any feedback!

  • 0
    •  Analog Employees 
    on Jan 14, 2020 8:35 AM

    Hi MrBlue

    I have a few inquiry regarding your issue.
    Regarding the channel mentioned on item number 2 (the bad one). What's the difference between this channel and the good channel? Are you expecting similar results on your system?

    For AD8421, in order see if this is the one causing the unexpected results.
    May we know the rest of the schematic on the input side of AD8421?  Also, what source are you using on the input and its setting?
    Can you try measuring the CMRR of AD8421 when the output is disconnected from the rest of the system?
    Knowing these information will allow us to help you out further.

    Thanks a lot MrBlue.

    Best Regards,