trouble with ai on 6259

[v1212800]

Hi,

I have some trouble measuring voltages on my PXI6259. When I measure the voltage with a digital multimeter, it shows 260mV. But measuring with the PXI 6259, the measured voltage jumps between 180 and 220 mV. And I read the voltage with the multimeter directly on the pins of my connector block (SCB 68) when it is connected. I'm measuring RSE. What could be the reason? Thanks for your help!

Tim

[Norbert_B]

Tim,

there are two issues which (can) decrease measurement-qualities.
1.) Range setting. the 6259 has a resolution of 16 bit. this resolution is devided evenly through the whole range. that means, the larger the range, the larger the error you are getting in result. if your range is set to +- 10V, the error is around 0.3 mV. as you can see, the error is about 0.1% of your signal which is quite huge, but not that big as you are seeing...
2.) i asume your DMM is a hendheld device. therefore, the DMM does not see any grounding issues. and that is, what i believe you are seeing. if your signalource is already grounded in any way, you should never use RSE. RSE is only usable with floating signalsources like batteries. in fact, if your signalsources is grounded (as i believe), you are lucky to see only an offset of around 80 mV. the offset can easily go up to several 10V...... to solve that issue, please just check the signal by connecting it differential and set the acquisition to the apropriate configuration.

Norbert B.
- NI Germany

[v1212800]

Hi,

thanks for the answer. Its not a grounding problem. I measured it differential with the same effect. But it seems that the signal itself is in this way and the multimeter is not fast enough to see it. I measured the signal also with a ground-free oscilloscope and I saw the noise as well. That means that the NI.card measures correctly. Is it possible to average the measured signal over a time, e.g. 0.5 seconds?

Thankls
Tim

[Norbert_B]

Tim,

i am glad to hear that the issue is not created by NI hardware 🙂
And sure, it is possible to average the measured values, but not within the hardware. When taking measurements, you will always get an array of the values back, even if the array ins contained within other datatypes (e.g. waveform or dynamic). Therefore (if not set to this already) you can always extract the array which contains your measured values over time. For your case for example, if your samplerate is 10kS/s, you would have an array of 5000 values representing 0.5s.
Every ADE should deliver functions for arrayhandling including an average-function for 1D-arrays.

hope this helps,
Norbert B.

[v1212800]

Thanks for the answer,

is there any example code for it?

[Norbert_B]

Tim,

this depends on the ADE (Application Developement Environment) you are using. NI delivers examples for LabVIEW, LabWindows/CVI, Measurement Studio and DIAdem but as well as ANSI C and sometimes .NET-only. But NI does not deliver examples containing the array-handling as well as data acquisition, at least not what you exactly are searching for.
I advice you to take a look into continouos acquisition examples as well as array examples and combine them in a way that fits for you.

Norbert B.