pxi6704 grounding issue

[zamith]

Hi,

I am using a pxi system PXIe1078 hosting a 16bit analog output card PXI6704.

The pxi6704 is connected to the SCB-68A connector via a sh68-68-D1 cable.

I use the analog outputs to drive high voltage power supplies. The high voltage power supplies are controlled by 0-5V input.

When I output 0V from the pxi6704 card, I observe the following:

- when measured between the analog output and the analog ground, the voltage output is exactly 0V

- however when measured between the analog output and the pxi chassis or cable grounding, the voltage is 4mV.

(note that these observations are made with nothing connected to the scb68A connector, I have checked that the same happens when the measures are made directly at the pins on the card itself)

So from what I can see, the voltage between the anlog output and the corresponding analog ground is exactly what I expect. However, this is not true anymore when comparing to the pxi chassis ground.

I am a bit surprised by this offset, since it is stated in the manual of the pxi6704 that: "Analog Output Grounds—Each ground pin is shared between one voltage and one current
channel. These pins are connected to the NI 6703/6704 device analog ground plane.
All NI 6703/6704 device ground planes connect to the computer system’s ground signal."

My high voltage power supplies have a common ground with the pxi chassis.

Since my 0-5V control convert to 0-5kV output, this 4mV offset causes my high voltages to be offset by 4V, which is not acceptable for my application.

Is this a know issue, and appart from using isolation amplifiers for each analog output (which I'd like to avoid), is there a way to fix it?

Thanks for any answers you may have.

Sincerely yours,

Sébastien Zamith

[zamith · Accepted solution]

Well, I got an answer from NI: apparently this would be a "known" issue.

The solution is to use insteatd nanalog output cards with isolated outputs, such as the PXIe-4322...