# Need help modelling the circuit with parasitic capacitance of a power supply in this scenario in SPICE

A single-ended source is connected to an inAmp of a data-acquisition board as follows:

As you see above, the shield is shielding the twisted shielded pair cable. The shield is tied only to the earth ground. AIGND is not connected to earth ground.

I want to make a very simplistic simulation in LTspice, to mimic this scenario when there is a 50Hz common mode noise comes through the power supply to both lines as common mode interference.

Below is my attempt:

There is something missing or wrong in above model.

I want to model the circuit where the common-mode interference comes through the SMPS power supply. Power supply need not to be modeled juts the noise like in my circuit. But there is also shield and the AIGND is not earth grounded.

How can I fix this to make it more realistic?

My second attempt:

edit:

• I've read this three or four times but I'm not clear about what you want. Where is the SMPS supposed to be situated? Jan 25, 2018 at 17:23
• SMPS not needed just a signal source which does the same effect of it. Suu Vs2 in the LTspice circuit. Imagine that is a transducer. And imagine the SMPS power supply of this transducer is leaking 50Hz common mode voltage all the way to the inamp lines. That is what I want to model. It is to convince some people the effect of it.
– GNZ
Jan 25, 2018 at 17:27
• I tried and added another attempt. I dont know whether is better.
– GNZ
Jan 25, 2018 at 17:29
• If the common mode voltage is high the inAmp outputs very weird. And 10k creates imbalance and makes it impossible to zero the 50 Hz noise.
– GNZ
Jan 25, 2018 at 17:32
• OK, your first attempt is OK if the SMPS is in the sensor but you will find that the series capacitance is more like 1 nF with very little resistance. At 50 Hz and 230 volts AC this can inject a current up to 70 ish uA. Jan 25, 2018 at 17:32

The two capacitors near the InAmp remove common-mode AC content caused by the 1 nF in the SMPS modelled to the left. Some SMPSs will have significantly less than 1 nF and may be tamed by 10 nF decouplers but other SMPSs might have 2.2 nF internally and will need greater help hence 100 nF or greater may be needed.

Bear also in mind that if 1 uF capacitors are needed thay will reduce the output bandwidth of your source because of the 100 ohm balanced drive.

Typically, 1 uF and 100 ohm produces a cut-off frequency of 1600 Hz. However, if you are only worried about slow speed signals from your source 10 uF capacitors can be employed.

One thing to watch though - those 10 uF capacitors will produce a differential signal if they are not perfectly matched.

• I think in this circuit on the right side Earth and the AIGND should be wired. Earth symbol is hand drawn lines. But I did that connection now and doesnt work man. Output jerky. I can tell you 1nF leakage is so small to see that common mode range effect. Make it 100nF and common mode voltage becomes around 40V and those parallel caps doesnt rescue. Try simulate maybe you can figure out why. Im not familiar as much as you.
– GNZ
Jan 25, 2018 at 18:32
• Alright 10u worked. Before I had tried upto 100nF only.
– GNZ
Jan 25, 2018 at 18:33
• You said in your question that "AIGND is not connected to earth ground" so I went with this idea. Are you changing your mind? Jan 25, 2018 at 18:43
• It is not connected in real, but for the sim to work it is connected through that 1nF capacitor on the left side. If you dont connect that on the roght there will be no loop you cannot capacitively couple the common mode voltage in the sim.
– GNZ
Jan 25, 2018 at 18:44
• Vinterefence is tied to earth but that is not ground in the sim, that earth symbol is just lines I drew
– GNZ
Jan 25, 2018 at 18:46