measuring DC voltage using NI-USB-6009

[JudeLi]

Hi guys!

I was measuring DC voltages using ni-usb-6009 and I saw somthing weird.

There were two channels, chiannel 1 is -9.7V and channel 2 is around 0V and there was a 10k resistance between 0V voltage source and the ni device. Both virtual channels were differential.

After I measured them with rate of 1k/s and 20k/s, the weird thing happened (see the attachment). Channel 2 is really unstable compared to channel 1, but it was normal when I took away the 10k resistance.

Then why did it happene? Not sure if it's a ghosting effect or not, if it is, shouldn't it be just lower, in stead of being unstable?

Best regards,

Jude

[johnsold]

The input network on the USB-6008 (as shown in the User Guide) has an equivalent series resistance of about 144 kohms terminated into 1.4 V.  At the higher speeds you are probably seeing effects of settling time, especially with the additional resistance.  If the other channel was grounded rather than being at -9.7 V, you would probably see a smaller effect.

Lynn

[JudeLi]

Hi Lynn,

Thanks for replying!

The problem is that the measured signal is also unstable when I set the rate to be only 1000S/s and, when I change the virtual channels to be RSE mode from Differential mode, It went back to normal.

Best regards
,

Jude

[Erik-S]

What pins are you using in your setup? I would double check your grounding to make sure that it is truly differential. Here is a link to a white page for field wiring which should be useful.

http://www.ni.com/white-paper/3344/en/

-Erik