# Is this definitely common-mode interference and how can I model and filter it?

A test device which can output voltage between 0 to 10V by a knob is powered by a DC adapter which converts 220V rms(meaning 311V amplitude) 50Hz AC to DC. Device is not a commercial and its adapter looks like a two prong laptop adapter.

When I use this device with any data type of acquisition(either single or diff-ended) I see significant amount of 50Hz noise. And when I put a very large cap like 47uF between its output terminals the 50Hz noise was mitigating but still annoyingly exists.

Then I do the following test with a multimeter's voltmeter settings: When I connect either terminal of the device to one lead of the voltmeter and connect the other lead of the voltmeter to the earth I read 80 VAC. Figure 1 and Figure 2 illustrates that. This 80V AC the voltmeter shows must be rms so the amplitude is around 113V.

If I connect the voltmeter leads to each terminal of the device as in Figure 3 for lets say 1V DC setting I read 1V DC and 0 VAC.

And there is also another multimeter which has a low impedance setting. So if I set the voltmeter to low impedance those 80 VAC in Figure 2 and Figure 3 almost disappears to zero.

So I'm almost(?) sure that there is common-mode voltages on each terminal probably caused by the leakage through the power supply.

I measured the output resistance of the device between 20 Ohm to 100 Ohm.

The input impedance of the voltmeter I used is 10Meg Ohm.

First of all I wanted to model this in a circuit before I ask my question as follows: I've chosen the parasitic capacitance as 250pF such that to make the Figure 1 and Figure 2 voltmeter measurements 80 VAC rms i.e 113V amplitude sine. I also add the output resistance of the device as 40 Ohm to see the effects.

Below are the plots for Figure 1, Figure 2 and Figure 3 situations: As you see above as the red plot the common-mode noise is not rejected well because of the unbalance above due to the source output resistance, there is 50 Hz noise on the 1V DC voltage.

My questions are:

1. Is my circuit model for the situation correct?

2. I tried to add some resistor Rx on the other line to match the balance in real but as long as the difference Rout-Rx is not zero I think there will be common-mode noise. In sim it is easy to match. I actually couldn't manage it in real since I only have fixed resistors.

So is it normally/practically done by using a potentiometer leads and adjust it very delicately? I heard about common-mode chokes but those are only for RF frequencies or still can be used for 50Hz common-mode coupling?

## 2 Answers

Yes it is CM line noise. simulate this circuit – Schematic created using CircuitLab

Try the nearest earth ground connection.

• Woow you are right in simulation at least it worked. I will try on real tomorrow. – HelpMee Jan 24 '18 at 20:58
• Answers with diagrams always the best answers, it prevents long comments. – HelpMee Jan 24 '18 at 20:59

This is a well known phenomena in SMPSs. To avoid a lot of switching noise on the output wires they connect a 1 nF or 2.2 nF from the low voltage dc output to the primary dc bus voltage after the bridge rectifier so, apart from a pile of innocuous (to most circuits) 50 Hz, there can sometimes be a percentage of 100 Hz. So remodel the 250 pF to be about 1 nF and maybe use a bridge rectifier and smoothing cap loaded to give 20 volts peak to peak ripple.

• Thanks regarding 2nd part of the question: if you dont have option to change the power supply and want to get rid of this for the data acquisition, is the resistor adding way practical or only in theory? How about choke? How would you solve this without using buffer or any active circuit? Or there's no solution? – HelpMee Jan 24 '18 at 20:18
• And if you can model this basically crudely and draw in your answer that would be great for lots of young players and for me as well :))) – HelpMee Jan 24 '18 at 20:20
• To get rid of it you tie one end of the dc output to the DAQ ground point. Or you use a 100 nF or greater to ground the supply. I’m not sure how to answer this because I can’t get inside your head. There are plenty of answers I’ve given over the years that show a circuit taken from a company called power integrations. Those answers usually are to questions like, why do I get a tingle when I touch the leads etc.. Not at pc hence really difficult to embed pictures. – Andy aka Jan 24 '18 at 20:36
• "100 nF to ground the supply" I will try this. You mean one leg of the cap will be at earth and where will be connected the other terminal of the cap? – HelpMee Jan 24 '18 at 20:40
• The other leg is on the supply dc output either + or -. – Andy aka Jan 24 '18 at 21:14