How to deal with differences between AO GND and pressure controller ground

[audreymoira]

Hello, I am using a USB 6343 to control an electronic pressure controller (EPC, Porter Model 415, http://www.porterinstrument.com/content/files/datasheets/Models-415,425.pdf).  As in the attached wiring diagram, the EPC is powered by a 15VDC power supply (LS25-15, https://www.us.tdk-lambda.com/ftp/Specs/ls.pdf).  The USB-6343 is using its own adapter, plugged into the same power strip.  However, the ground of the power supply and the AO GND have a fluctuating voltage difference as large as 9 volts.  So this creates problems in setting the 0-5V pressure setpoint using the DAQ's AO, since the DAQ is referencing AO GND, and the pressure controller is referencing the power supply ground.  I can't just use two AO's given the magnitude of the voltage difference (would sometimes be out of the DAQ range). 

From what I've read, it would not be sufficient to connect AO GND to the power supply ground (and this could cause damage?).  Here are the two ideas I had--any recommendations? 

1) How could I tie together the power supply ground and DAQ grounds?  If it helps, I could switch out the TDK-Lambda LS25-15 linear supply with an Omron 24 VDC supply (S8VK-G12024, http://www.ia.omron.com/products/family/3178/specification.html).  (I tried this swap, and it didn't help...still had voltage differences of up to 5V.)

2) Alternatively, it looks like I could use an instrument amplifier (such as AD8422 https://www.analog.com/designtools/en/diamond/?doc=AN-1401pdf#difL=0&difR=5&difSl=5&gain=1&l=-9&pr=A...) to add the power supply ground and the AO voltage?  I've sketched this out in the wiring diagram.