USB-6363 BNC board Grounding

[e1010511@u.nus.edu]

Dear all, I have a silly question: could anyone help me find the analog ground interface of USB6363 BNC. Below is my question details.

I was using USB-6363(BNC) with AI0:1 & AO0. AO0 is connected to AI0 directly, and also connected to an analog low-pass filter (100Hz pass band), which is powered by a DC power source. The output of LPF is then wired with AI1. But when I was testing, I set AO0 as 0V, while AI0 & AI1 will read a signal with 50Hz power frequency interference. I assume it was because I didn't connect the USB-6366 ground with the DC power source ground. Is it the reason for the 50Hz noise?

Thanks.

[NathanDavis · Accepted solution]

Hello e101,

The first thing that comes to mind is whether your toggle switches on the BNC board are set to FS or GS? Since you have your grounds tied together, toggle the AI channels to the GS state.

https://forums.ni.com/t5/Volume-License-Manager-and/When-will-we-switch-on-FS-GS-when-using-NI-USB-6...

If the AO0 and AI0 are shorted together, what does the low pass filter (LPF) connect? Is there a "T" junction leading into the AI0 input as well?

Most certainly not connecting your power supply ground reference to the BNC reference would be an issue. You must close the circuit, otherwise parts of your circuit will float at odd potentials.

Also, how do you know this is 50 Hz noise? Have you connected the output of the power supply to an oscilloscope to confirm? If you AC couple the oscilloscope, the DC voltage will vanish leaving you with any frequency dependent signal. This will show any leaking 50 Hz noise from the wall voltage, though this signal should be greatly reduced from the power supply filtering.

Consider also using a battery in place of the power supply. Just for testing purposes, hook up the same arrangement, and see if the 50 Hz is still being picked up. If 50 Hz is NOT present, then you know the power supply could be leaking some small signal 50 Hz on your power supply line (and the 100 Hz LPF wouldn't filter that any way).

[e1010511@u.nus.edu]

Hello NathanDavis,

I tried several methods last week and found that the reason hide in the power source! Here's what I found:

1. I have toggled all the AI channels to GS.

2. There is a T juction on AO0, one to LPF and another one to AI0, while the LPF output to AI1.

3. I tried connect all the grounds together but it wouldn't work.

4. I use "FFT" function in labview to analyze the amplitude-frequency of the signal. Also I used an oscilloscope to confirm it's a 50Hz noise. But when I output AO0 to control the device, 100Hz, 150Hz, 250Hz, 350Hz & 450Hz noise appeared. 

5. I tried battery source and the noise greatly decreased. So I tried hard to decreasing it using DC power supply by shielding it with a metal box and other methods, but nothing works. I will try some other to solve it. Thank you so much!