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This is a question I wanted to ask after I have observed much more noise when I swap a filtered traducer's output to a single ended data acquisition board. A transducer outputs Vs1 signal. The transducer is basically a strain gauge with an amplifier and powered by an SMPS supply.

So in Figure1, Vs1 is the transducer output signal, Vdn1 is the differential noise and Vcm1 is the common mode noise. R1 and C1 forms a low pass filter. Rin is the input impedance of the data acquisition input.

enter image description here

In Figure2 everything remains the same but the inputs to the DAQ are swapped.

When I simulate this, I get much more common mode originated noise in Figure2.

Is that correct to say for such circuit setup in Figure2, if we swap the inputs common mode noise component will increase. Am I modelling the issue and interpreting what happens correct?

Edit:

enter image description here

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  • \$\begingroup\$ "The transducer is basically a strain gauge with an amplifier and powered by an SMPS supply." If aiming for low-noise operation, the SMPS may be a source of noise. \$\endgroup\$ – rdtsc Jun 26 at 12:10
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This answer was given prior to the OP changing the question following the observations I made in my answer directly below.

Is that correct to say for such circuit setup in Figure2, if we swap the inputs common mode noise component will increase. Am I modelling the issue and interpreting what happens correct?

In figure 1, the common mode noise is shorted to ground hence there is no common mode noise to interfere with the input. In figure 2, the common mode noise source connects directly to the input. What do you really expect under these circumstances?


EDIT - answer to edited question

Scenario 1 applies somewhat less than 50% of the common-mode noise to the input via a 3.3 uF capacitor. Scenario 2 applies somewhat less than all the common-mode noise to the same input (C2 now grounds some of the CM noise). If you want to show it correctly on your transient graphs, reduce your differential voltage to zero volts and re-plot.

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  • \$\begingroup\$ I forgot to add milli-ohm level resistances for the ground trace. In Figure1 if I take ground wire resistance as 10 milli Ohm, Vcm1 goes up to 1V. So something in the model might be wrong. \$\endgroup\$ – cm64 Jun 26 at 10:31
  • \$\begingroup\$ I added realistic ground resistor and added plots to show what I meant more clear in the Edit. \$\endgroup\$ – cm64 Jun 26 at 10:36
  • \$\begingroup\$ @cm64 - be aware that I'm not going to do any more iterations of your evolving question unless it is absolutely vital and important. Look at your circuit and figure things out more logically. If you don't understand my answer just ask but don't start moving the goalposts anymore please. \$\endgroup\$ – Andy aka Jun 26 at 11:00
  • \$\begingroup\$ Woow now I got it. Milliohm resistor and 3.3uF at 50hz forms an RC divider. and 1m Ohm is too low. If I increase the cable resiatnce to 10mOhm. Common mode noise of Vout1 becomes much much lower. Thanks for the tip! \$\endgroup\$ – cm64 Jun 26 at 11:29

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