Digital Multimeters (DMMs) and Precision DC Sources

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50 Hz interference PCI 4070 DMM

Hello,

 

I am using a PCI 4070 DMM to measure a waveform voltage in a range of 200 mV at 1 kS/s. The point is that I see 50Hz interference which is higher than the useful signal I need to acquire.

 

As I learnt from the DMMs Help file:

 

Note When using a resolution of 6.5 digits on the NI 4065 and NI 4070/4071/4072, 50 Hz and 60 Hz noise will be rejected automatically. This is due to the selection of default aperture times on the devices, which appropriately filter out noise.

 

Since I set sample rate at 1 kS/s, 50 Hz noise is not rejected automatically; I tried configuring powerline frequency at 50 Hz but the problem remains, so I would like to know how I can filter out noise. Maybe adjusting aperture time or other parameters in a properly way?

 

If it’s not possible through DMMs measurement configuration, should I turn to signal processing tools?

 

Thanks,

 

Marc

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Hi Marc, 

 

What type of signal are you measuring, how high is the frequency content of interest? Are you getting an effective sample rate of 1kS/s or are you configuring an aperture of 1ms?

 

Here is a link that better describes the DMM measurement cycle (http://zone.ni.com/reference/en-XX/help/370384N-01/dmm/measurement_cycle/). I wanted to be sure you were aware of all of the parameters that affect the effective reading rate of the DMM. 

 

Your DMM bandwidth, assuming a 1ms aperture time, is wide enough to measure the 50Hz interference. If you increased the aperture time for the DMM you would reduce that bandwidth and begin reducing the amplitude of the interference. (http://zone.ni.com/reference/en-XX/help/370384N-01/dmm/dc_noise_rejection/). The best rejection is achieved when your aperture time is configured to be at 1 PLC, however this will result in less bandwidth for you measurement. 

 

If your signal of interest requires you to have an aperture time of 1ms, you could post process the data with some filtering. 

Steve B

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Hi Steve,

 

Thank you for your repply.

 

I am measuring an analog signal, a transient potential which occurs in a couple of seconds as a maximum. So I set a sample rate of 1kS/s without configuring aperture time. I have tried increasing aperture time, but the amplitude of the interference is still larger than the signal.

 

I look at the filters in signal processing palette, but I am afraid that kind of filters could remove other components which are useful for my measurement. Is it possible to remove a single frequency (50Hz in this case) from the signal with any function in Labview?

 

Thanks,

 

Marc

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Hi Marc, it sounds like you may be using the waveform mode of the device for your measurements. If you are not able to sample more slowly, like 50S/s, you could try to reduce the 50Hz interference and post process the data.

 

Using short, shielded, twisted pair wires, keeping away from noise sources (power line, lights, motors, etc) would be my recommendation.

 

For post processing there are tools available in LabVIEW for filtering. You will want to be sure that your data is in a waveform data type such that it has dt information for the filter. You can then look under Signal Processing>>Wfm Conditioning and to begin with you may want to try the Filter Express VI (http://zone.ni.com/reference/en-XX/help/371361J-01/lvexpress/signal_filter/). This may require the Full Development System installation of LabVIEW. 

Steve B

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