Pseudo-differential input noise

[Aorta]

Hello Everyone,

I am using PXIe 4480 daq card with PXIe 8301 remote control module and PXIe 1071 chasis. When I record data with PXIe 4480 with pseudo-differential ground option, I am observing somewhat periodic noise. To trace back the origin of the problem, I removed the cables and recorded the signal. My sample rate was 1.25MHz and I took 1.25M samples with AC coupling. Afterwards, I took the FFT and calculated the frequency response. The noisy frequency spectrum can be seen in below. I also measured the same response in different rooms, which indicates that it is not an environmental noise. I don't understand why I am measuring frequency peaks. It should be white noise entirely. If I use differential grounding option, I don't observe this kind of behavior. However, I have to use pseudo differential grounding for my experimental setup. Does anyone have any idea why pseudo differential grounding gives this type of noise, and how can I eliminate these frequency peaks? Thanks in advance.

Kind regards,

Adil

     

[ZYOng]

Excerpt from Incorrect Readings on Unconnected or Open Channels of DAQ Device

NI does not specify what voltage a device will read when there is no signal connected.

Since an open or unconnected channel is not being actively driven to any specific voltage you cannot expect a specific voltage to appear on the channel. To see zero volts on a given channel, there will need to be a zero voltage signal applied across the + and - terminals of the channel, which is the same as grounding the channel. To pull down the signal to zero, you can use a pull-down resistor. See Can I Use Pull-Down Resistors for Analog Inputs? for more details.

[Aorta]

Thank you for your answer. I am aware that "NI does not specify what voltage a device will read when there is no signal connected". However, I am observing these no signal connected peaks when I conduct my piezoelectric transducer measurements as well. Somehow, these peaks show itself during measurement, where I expect it to be noise floor. My guess is pseudo-differential grounding alters frequency response (with additional frequency peaks) and I am not sure the reason behind it.

[Yi_Fan]

I guess the sound and vibration might pick up noise from the bottom intake fan. You can try on rear intake fan chassis and see if the periodic noise gets lower or goes away.

https://knowledge.ni.com/KnowledgeArticleDetails?id=kA00Z000000kGeJSAU&l=en-MY