NI USB-6002 gives noisy reading even if bias resistors are used

[yxf5000]

Hi all!

 

I'm trying to use ni usb-6002 to measure a low level DC signal which is around 1.3 V.  I used differential set up and two 47k bias resistors. The device is connected to a laptop computer that is connected to a power outlet and is then connected to a wall socket. Here is an actual picture of the connection

 

 

However, this gives me really noisy measurement.

 

 

 

the actual signal should be about 1.25V.

and an fft image of it

 

 

so it looks like I get all kinds of interference.

 

However, if I unplug laptop's power, the measurement is much better

and an fft image

 

I mean if I unplug the power the measurement is much improved but still noisy. I could apply a low pass filter of really low cut-ff frequency and high order, but this would give me unbearable delay and defeat the purpose of real-time monitoring. 

 

So how can I take care of this noise problem? Any help is much appreciated!

[JKSH]

Your last FFT image shows that most of the noise content is in 50 Hz and its harmonics. I'm guessing that you live in a country with 50 Hz mains power, and the electromagnetic radiation is being picked up by your system. You wrote that the system "gives noisy reading even if bias resistors are used" -- Why would you expect the bias resistors to reduce noise? If anything, those long, unshielded resistor leads make them even more sensitive to radiation.

Ways to reduce electromagnetic interference include:

• Using shielded and/or twisted-pair cables.
• Using shorter cables.
• Checking your grounding.

 

Your first plot shows huge voltage drops from 1.25 V down to 0.2 V. I'm guessing you have a group loop or short-circuit somewhere in your system when your laptop's power adapter is connected.