bad readings with NI USB 6009 DAQ

[sb_oxinst]

Hi

I have a NI-USB 6009 DAQ device which I am trying to read a differential voltage across a 1ohm resistor. Everything seems to work fine but my readings are way off and +-30mV of noise. The specifications for the NI-USB 6009 mention that you can get pretty good resolution when using differential measurments. My readings should be 1.4V and instead I am getting something like 0.2V. I drew a simple diagram of my setup (see attachment) so if anyone could let me know what I am doing wrong that would great! thanks 🙂 

[AdnanZ]

I would connect some resistors. Have a look at Table 1 of the following article:

Field Wiring and Noise Considerations for Analog Signals

[AnalogKid2DigitalMan]

Shield drain wire should terminate to earth ground, not AI-. Any noise shunted by the shield would likely end up in your signal they way you currently show it wired.

-AK2DM

[sb_oxinst]

adding the resistors did not make any difference. I actually had checked out that article before but it was a good reminder. When I read the voltage across my analog input terminals (AI 1+ , AI 1-), it does read the correct value but the display on my front panel is showing a totally different number. I tried simply running MAX in differential mode and it gave me the same readings.

[SnowMule]

The signal you're trying to measure is way down in the dirt.  Of course it's going to be noisy. 

Amplify the voltage drop across the resistor before pumping it into the DAQ.  I like the MAX4372 amplifiers.  With that amplifier and a 1-ohm resistor, you'll get a pretty decent 0-100mA:0-5V reading.

[sb_oxinst]

i see. I have an AD620BR Instrumentational amplifier so I'll give it a shot. I'll let you know how it goes. thanks!

[sb_oxinst]

I still don't understand why I get 1.4V across AI0+ and AI0- (which the value I want) but when I read the voltage with MAX (ver. 5.1), it shows up as -270mV. As the voltage increases across the terminals, the readings get less negative and vice versa. Someone please explain!