PXI-6289 Simple Voltage Measurement

[Newbie217]

Hello,

I have a multifunction DAQ, the PXI-6289 connected to a BNC-2110 breakout box. I have BNC cables connected to the box that split into alligator clips on the other end.  I am trying to setup a LabVIEW VI to take some very basic, precision voltage measurements. I noticed that when I configure the measurement using the NI supplied  DAQmX drivers, I get really inconsistent voltage readings that flucuate greatly between measurements. For example, I am trying to read a reference at 1.24 V, and with the PXI-6289, I get anywhere from 1.23-1.25 V. I have tried configuring through NIDAQ Asistant as well, still no luck. I have tried changing the terminal configuration from Differential to RSE and NRSE as well. Is there a way to set, or configure the input resistance?

In comparison, I also have the PXI-4071 Digital Multimeters hooked up in my setup. When I take voltage readings with the DMM, I get rock solid consistently that verfies nicely with a standard benchtop unit. Ideally, I would really like to be able to get the multifunction DAQ working as well so that I can maximize the number of inputs avaiable. Any help is appreciated. Thanks for your time!

[Christian Mergl]

Hey,

With your code the bits of the ADC are spread over the whole range of the card, which should be -10V to +10V, so you have a loss in precision. You should configure the limits to be from +1 to +2 Volts if you wanna measure this 1,24V with a higher precision.

Antoher thing is, why do you create your controls for the channels and the error and then use a variable to connect it with the DAQmx Create Channel. I assume this are your first steps with LabVIEW and you are familiar with textbased programming languages. You should have a look at the provided examples for DAQmx.

You will find them either with the LV Example Finder under the Help menu or under [LVDIR]\examples\DAQmx\

Christian

[Dennis_Knutson]

I would also recomend taking more that a single sample and averaging them to get something closer to how a DMM works.

[Newbie217]

Thanks guys! I failed to realize that I was only taking one measurement. Taking multiple measurements (500 or so) and averaging them out tends to give pretty good accuracy, plus or minus 3 mV or so. Still not up to par with a DMM, but much better than before. I tried playing with the min/max ranges as well, but they don't seem to do much.

Any suggestions on how to go about current and resistance measurements? Are shunt resistors needed when using these multifunction DAQ's or is there a built in, internal shunt resistor I can access? Thanks a bunch.

[Christian Mergl]

Hey,

You will need an external shunt resistor.

Christian