Important Noise Probleme measuring a 450mV signal with NI card

[marc france]

Hi everyone,

I am using Labview 8.0 and a NI DAQPad-6015 card. I am measuring a continuous voltage of about 450mV. I have taken a photo of the noise I can see on a oscilloscope (photo n°1). I roughly get about +/- 4 mV . That’s ok in my situation.

However when I want to measure this voltage with my NI card and Max I get so much more noise (photo n°2), nearly +/- 50 mV !!!  I have put a 10Kohms resistance between AIground and V-, and also between AIGround and V+ ( I have also tried without any, and with only one but no diffrence). I have got exactly the same result when I use my other NI card PCI 6221.

Would anybody know what the problem is and how I could solve it ( I am not looking for an average solution for example but a NI solution on how to measure this voltage)? I must be doing something wrong …. I have used the differencial mode because of the noise I had already seen on the oscilloscope. For all measures (oscilloscope or NI card)  I use the same cord (about 30cm long). I have taken an example around 450 mV but the voltage can go from 0 to 5V

Any help would be great,

Thanks,

Regards,

Marc

[johnsold]

Marc,

Measuring noise can be tricky. The bandwidth and sampling rate are likely quite different between your oscilloscope and DAQ card measurements. The noise power is a function of the bandwidth, so the measurement bandwidth must be the same for each system or you must compensate for the difference. The sampling rate(s) must satisfy the Nyquist criterion for each system. Without more details about the signal and noise source and the measurement bandwidths it is dificult to tell whether you have a problem or are measuring two different amounts of noise.

Lynn

[gwd]

Are you sure you haven't got the X10 probe button on the oscilloscope pushed?

[marc france]

Hi Lynn,

Thanks for answering my question. I have connected the ground connexion of the NI card to the ground of the supply voltage of the circuit. I now roughly get about +/- 5 mV like with the oscilloscope.

regards,

Marc