AI problems w/ New USB-6421 boards

[djb]

We just purchased two new USB-6421 boards.  When I run then in test panels or in Labview using one of the example programs the AI voltage readings act like they are 'shorting out'...i.e. when I connect a channel to a constant voltage source like a battery I get some readings at the proper voltage and some readings at or near zero.  The problem is only occurring on one of two computers I've tried it on (one laptop one desktop)....Both computers are using the latest driver (NI-DAQMX 26.3) and running Windows 11.  On the laptop I get the expected behavior, a quiet signal at the proper voltage, but on the desktop I get this: 

I've tried multiple cords and different USB ports but get the same behavior every time.  On the same desktop machine I get good results from a USB-6002, so the problem is limited only to these new USB-6421 modules.

Any ideas on what might be causing the problem or how it can be corrected?

[santo_13]

Please share your connection diagram. Please ensure you follow How to Connect DAQ Device and Analog Signal Source - NI.

If you're connecting a battery, please follow Floating source connection details.

[djb]

I'm just connecting a Fluke voltage simulator to the ai0 input (positive label 0, negative label 😎 in differential mode.  As I mention in the original post, that connection method works fine on the laptop computer using the same USB-6421 module (with the same connections), its just the desktop where I see the ugly trace.

I should mention that I've tried reinstalling the NI-DAQ driver, but still have the same problem.

[djb]

that icon should be an eight

[santo_13 · Accepted solution]

---

@djb wrote:

I'm just connecting a Fluke voltage simulator to the ai0 input (positive label 0, negative label 😎 in differential mode.  As I mention in the original post, that connection method works fine on the laptop computer using the same USB-6421 module (with the same connections), its just the desktop where I see the ugly trace.

I should mention that I've tried reinstalling the NI-DAQ driver, but still have the same problem.

---

So, you have clearly validated the importance of proper connections when using a floating source.

When using a laptop that is battery powered and not connected to mains, you avoid common ground and everything is perfect, the moment, the DAQ is ground referenced as when connected to a mains powered computer, you will see the effect of leakage currents and the importance of the bias resistors.

please read How to Connect DAQ Device and Analog Signal Source - NI

[djb]

Thanks!  The bias resistors do solve the problem.  I've been using USB-6002 modules for years (in differential mode on desktop computers) and never needed the bias resistors...but definitely needed here.

Does anyone have an elegant solution for wiring all the available channels with bias resistors?  That seems like it will be a wiring nightmare.