Poor CMRR Results with AD620

I've been working for a while with the AD620 for a differential audio application. One test I'm using to determine how effective the circuit is at rejecting common mode noise is to directly couple my signal generator to both inputs of the AD620. I'm feeding a 500 mVRMS 1 KHz signal into both inputs and listening/measuring the output of the AD620 to see if it's attenuating it as much as expected. Unfortunately I can easily hear the 1 KHz tone in my audio path.

I realize the datasheet spec's for CMRR are based on DC to 60 Hz, but based on Fig 14 of the datasheet, I should still be able to achieve 80 dB of attenuation at 1 KHz. So far I'm not getting anywhere near that. The circuit I'm using looks like this. I've tried stripping out C64 and C65 and it helps, but the result is still nowhere near -80dB. I'm the one that layed out the PCB, and I'm pretty confident (although not positive) in the grounding that's being provided since I know that can be a factor.

Any insight would be appreciated.

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    •  Super User 
    on Mar 16, 2014 5:34 PM

         Byron,

         Where I work we're very happy with the SSM2019 -- although I haven't actually measured its CM response, this mic preamp has proven itself in installations with 200' cable runs.  I do recommend enough input RF filtering to keep out nearby AM stations.  We use 390 uH surface-mount inductors with 1 nF to gnd.

         Best regards,

         Bob

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  • 0
    •  Super User 
    on Mar 16, 2014 5:34 PM

         Byron,

         Where I work we're very happy with the SSM2019 -- although I haven't actually measured its CM response, this mic preamp has proven itself in installations with 200' cable runs.  I do recommend enough input RF filtering to keep out nearby AM stations.  We use 390 uH surface-mount inductors with 1 nF to gnd.

         Best regards,

         Bob

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