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AGND/DGND decoupling using eval board

Question asked by Andrea111 on Dec 21, 2014
Latest reply on Dec 30, 2014 by musach



I was wondering if someone could help me with some best practices.  I am not an engineer by training, but need to build some low noise electronics for cryogenic measurements of devices.  I put together a DAC/ADC data acquisition system using ad ad5764 eval board, ad7732 eval board, and an arduino DUE microcontroller.  I am communicating using the SPI interface between the arduino and the two cards.  My question concerns isolating digital from analog ground.


On both the eval boards, there is an exposed gold trace on the back that appears to be the only connection between AGND and DGND. Currently, I am running the analog sides of both cards off a regulated power supply and eventually will use batteries, while the digital sides are powered by the arduino, which is in turn powered by the computer.  I want to make sure I isolate the measurement gnd from the computer.  My questions are


1) When I cut the gold trace on the AD5764 eval board, the AGND-DGND resistance reads open.  However, when I do the same for the AD7732 eval board, I still get 20 ohms or so.  Am I missing something here? 


2) Is cutting these two traces even the right thing to do?  The AGND will not be defined by the chassis, which is connected to the center of my grounding "star" (in this case, my cryostat) by the shields of the bnc cables.  Is this likely to give good noise performance? 


Many thanks in advance,