Can I use differential ground with PXI 6704?

[User002]

I attempting to use DAQmx and the AO pins of the PXI-6704 to output a current.

I have attached some conflicting information I have read in the User Manual for the PXI-6704. One page indicates that I can use either floating or grounded load when the device outputs a current, and the other page indicates all the AO pins are connected to the computer system's ground. Only the latter seems to be true.

I have also attached a photo of a probe measurement on the SCB-68 at AO ground pin 68 (channel 16), attached to the PXI 6704, which is powered on but not active in the photo. Between the body of the SCB-68 and the AO GND pin 68, I find a very high resistance. The AO GND pins are all grounded.

Has anyone used the PXI-6704 for an Analog Output current with floating ground successfully? Is there a setting or property in Labview that I need to use to have floating ground?

Thanks,

I.G.

[AirFrantz]

I'm a little confused by your question.  The manual indicates that you can connect to a load that has a floating ground, not that the current source can have a floating ground.  All of the AO GND pins are indeed connected to the computer ground.  To connect to a floating load, you would simply need to set up your circuit as in figure 2-4 from the manual.  There is nothing to configure in software.  If the load was grounded, I believe you would need to connect the load's ground to the AO GND pin.

[User002 · Accepted solution]

The answer is no.

I spent the better part of the day on the phone with NI engineers. The PXI-6704 manual provides conflicting information. Indeed it informs the PXI-6704 operator that she "can connect to a load that has a floating ground," but it should also mention that the load will no longer have a floating ground, because the device ground planes are connected to the computer system's ground signal, which is connected to earth ground. I'm not sure why the verbiage about floating ground is included because there is no way to isolate the load with this device.

Our team returned the PXI devices to NI. The Application Engineer we worked with has spoken to R&D about clarification in the manual.