0
\$\begingroup\$

Reading this documentation, what could be the reason to use 10k if one uses differential ended input settings for signal sources?:

enter image description here

I dont know why they recommend 10k but I had 6 channels going to this board from single ended 6-axis force transducer system and the signals measured were very clean. But when I added some other transducers to rest of the channels such as temperature, humidity ect. transducers, I started to have more noise and 50Hz noise. All inputs to the daq are single ended( the transducers' grounds are not earthed so no ground loops expected) and BNC coax cables used.

In my case I had shorted all channels' CHLow to LLGND but they recommend 10k resistor, could that be the reason? What could be the 10k resistor's function?

edit:

I tried to make a simulation regarding Spehro Pefhany's answer:

enter image description here (left click to enlarge)

I couple EMF to both lines, and Rx is the recommended 10k.

If I dont use 10k and short CHLow to LLGND I obtain noisy output(red plot): enter image description here

If I use 10k the 10k develops a common-mode and the output(red plot) doesn't have the noise:

enter image description here

I haven't tried this 10k in practice yet though.

\$\endgroup\$
  • \$\begingroup\$ You MUST provide some current into the two input pins of the amplifier. Perhaps only picoamps if the amplifier is FET-input, tho at 125 degree Cent that input current will increase 1,000X if not 1,000,000X. \$\endgroup\$ – analogsystemsrf Dec 21 '17 at 2:29
2
\$\begingroup\$

The resistor establishes a common mode voltage that is within the range of the amplifier.

You could also use two 10K resistors, one from each input line to LLGND. That would load the signal differentially but would retain balance. Grounding one side or using one resistor does not load the differential signal but makes the input more like a single-ended input (in the case of grounding it, exactly like a single-ended input) so you lose some of the advantages of a differential input.

The value of 10K is not magic. Values as low as 50\$\Omega\$ are used sometimes- just enough to prevent serious ground loops in the case where the grounds are not isolated from each other. That's called a pseudo-differential input.

\$\endgroup\$
  • \$\begingroup\$ Thanks and please see my edit. I made some simulations regarding your answer. Let me know if I understand the concept and what you mean correct. I haven't tried this 10k thing in real though. I will first try 10k to LLGND first because it only requires single resistor. Should I make star connection to the LLGND/AGND of the daq board or daisy chain would be alright? I currently daisy chain all CHLow together. to a point and from there 10k to the LLGND. I can draw if I didnt explain this clear. \$\endgroup\$ – atmnt Dec 20 '17 at 14:51

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.