m3atwad

Help with dropout at amp input when ADC starts

Discussion created by m3atwad on Oct 7, 2016

Hello,

I'm not sure this is actually the best place so, admin, feel free to move if there is a more accurate section of the forums.

 

I have an AD8237 amp that has capacitively coupled inputs.  With my product, I kick of and ADC and write the data to memory.  I am seeing this dropout when I kick of an ADC recording duration:

 

 

I am using a B&K precision sig gen to generate a 1khz sine wave.  I thought this was a problem due to my amp, but I now see that if I disconnect the sig gen from my data recorder and just connect the sig gen's output to the O'scope I see the same shift there.  The top two plots show my two ac coupled AD8237 inputs and the third one is a sallen key filter.  For some reason it bounces in the opposite direction. 

I have a 3.3V power plane for all my digital stuff and when the ADC and memory hit it drops a little.  This isn't a huge problem though because all analog circuitry is on a different power plane that is generated by and LDO and it is very clean and doesn't have any drop out issues. 

 

 

The top (blue trace) is my unregulated 3.3V plate.  This is really just in here to show that the input drop coincides with kicking off my sampling and memory write etc...  They bottom (yellow) trace shows the output of my signal generator.  The problem is that when the signal generator is merely grounded to the product and NOT driving any channel inputs I still see this drop.  Could this be due to a ground shift?  Everything in the device is tied to the same ground plane.  Do I need to separate the analog and digital ground planes?

 

Long story short I see this issue when my sig gen is merely grounded to my product....I can't seem to make much headway here and my entire analog front end is all analog devices amps so I figure I mine as well ask here.  Any comments or ideas would be greatly appreciated!!

Outcomes