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AD8237/ADA4661 high gain causes crosstalk

Question asked by m3atwad on Jul 21, 2015
Latest reply on Jul 23, 2015 by harryh

Hello,

 

I have a problem I'm seeing in my adc data.  In case this is an obvious mistake on my part I'll just describe it in general terms.  I have four adc channels connecting to programmable gain analog circuits involving the op-amps listed in the subject.  Channels 1 and 2 have an offset of 1/2(Vcc) so when there is nothing connected to channels 1 and 2 the ADA4661 drives ADC channels 1 and 2 with ~1.65V.   When I look at the data from the adc it is right on the money.

 

When I ONLY input a signal in to channels three and four, but record across all four channels (channels 1 and 2 being driven to 1/2(Vcc) I see the signals from channels 3 and 4 as if they were coupling into channels 1 and 2.  The weird part is that this is related to the amount of gain I apply.  So when I apply no gain it's not a problem.  If I apply a gain of 50 I can see a small version of channel 3 and 4's input signal in the data for channels 1 and 2 where I should see the sampled values for 1/2(Vcc).

 

Could this be a lack of bulk capacitance or something like that?  Where the larger the gain the worse this problem gets because my power rail can't respond fast enough?

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