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NI 6221 gives 43kHz (times integers) in DC measurement

Hi, I'm now doing DC measurement of Hall voltages by using NI 6221. But I saw a series of Noise with frequency of about 43kHz (times integers) in an oscilloscope connected to the BNC2111 connectors. This bothers me for about a month already. Does anyone know the reason or the way to fix it? 

All my thanks

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Message 1 of 7

I have a few questions in order to better understand your noise issue:

1. What grounding configuration are you currently using? 

2. Is the noise occurring with multiple sensors?

3. What is the frequency and magnitude of the signal(s) of interest?

4. What is the magnitude of the noise?



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Message 2 of 7

1. I'm currently using RSE, but the signal comes out no matter in NRSE or RSE grounding. 

2. That signal happens in every channel.

3. The frequency is 43kHz x n, while n is an integer and magnitude is about 50mV.

4. about 50mV.


I wonder if that signal is the sampling process of A/D conversion. In which the DAQ is sending out a periodic delta signal to do convolution with the signal coming into it for sampling, so picking out signal in my analog signal (which is DC voltage from my Hall bars in a cryostat).


Another question is: what kind of voltage should I pass into the DAQ in order to get accurate measurement? I'm now using some amplifiers in a DIY circuit box to doing differential amplification of the Hall voltage and then picking up by NI 6221 DAQ in my PC.


Here's some results when I hook the scope directly into the DAQ BNC-2111 connector without connecting any sensor signal into it. 



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Message 3 of 7

The signal of interest after amplification is about 1V.

With all my thanks

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Message 4 of 7



I have a few more questions for you:


1. For clarity, the noise is occurring at the same frequency and magnitude as the signal you are trying to receive?

2. What are the two signals that are being displayed on the scope?

3. What are the internal and external clocks you're referring to in the image descriptions?




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Message 5 of 7

Hi Mike

1. Our signal is DC Hall Voltage from the Hall bars we are using in a cryostat. So it should be DC signals.

2. I thought that peaks in the scope is the sampling signals from DAQ, but I'm not that sure because I don't know how the DAQ works. 

3. Internal and External clocks modes are chosen from the DAQ Assistant in LabView Module.

All my thanks


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Message 6 of 7



If the signal you are attempting to receive is a DC signal, then I suggest implementing a low pass filter to remove the unwanted noise.

I've linked to a couple of low pass filter examples below:



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Message 7 of 7