Electronics tutorial - Filtering unwanted common mode noise from your oscilloscope measurements

hello and welcome back so what I want to talk to you about today is noise and in particular noise present in measurements now one of the most annoying things that happens when you're making a measurement is that you end up measuring noise that's not really generated by your circuit it's generated by some external factors and the problem is that you end up thinking that your circuit is noisy when all along the noise was coming from somewhere else let me give you an example so what I got here is my second channel terminated with 50 ohm resistor and my first channel connected to a circuit now at the moment both the channels are showing roughly the same amount of noise so somewhere below 10 millivolts now when I take my power supply and only connect the ground of the power supply to my circuit I end up measuring more than 30 millivolts of peak-to-peak noise which is not generated by my circuit but rather it's coming from the power supply so this is common mode noise coming from the power supply and affecting any measurement that I will try to perform so what I want to do today is look at the method to reduce this noise without actually modifying any of the equipment so if you're curious how we can do that and much more than keep watching [Music] now in a previous video I showed you how you can modify your power supply to reduce this noise but that is not always don't recommend the thing to do now to understand how this other fix will work let's first look at the problem again so what I got here is a basic simulation in which I'm modeling the electrical grid I got my laboratory supply which is supplied with both line neutral and new protected earth lines I got my input filter I got some sort of load on the input then a supply and the load on the output I inserted my common mode noise which is generated by the switching action of the switching supply in between and you got some extra capacitors on the output now normally all of this common mode noise should close through these output filtering capacitors back through the input capacitors into the circuit so it shouldn't go anywhere but when you connect your auto scope probe to this so you take your hasta scope round and simply connect it to this circuit then a new circuit appears so this common mode noise doesn't just close through these filtering capacitors it closes through the ground of the hasta scope which is directly connected to the protected Earth and back into the circuit again so the main problem is that the common mode noise is escaping through the ground line and then coupling into the protected Earth of the Osler scope now one of the ways to improve on this which I looked at in the previous episode is where you increase these capacitors so that they provide a much lower impedance track and you can also add a common mode filter on the output but as I said that involves modifying the circuit so another thing that you can do is provide a different link through which the common mode noise can go to the protected Earth and now one of the things that you will observe on any austere scope is that the ground of all of the inputs is interconnected but usually you'll be using it for probes but you also have an external trigger input which is not very commonly used and you also have this calibration output so this is a calibration signal that you can use to calibrate your probes but this also has a ground pin so what I want to do is take the ground directly from the AUSA scope and connect it to the power supply through a shorter line and try to reduce the noise this way and the reason why that should work is because the voltage that's dropping on the AUSA scope probe is mainly caused by its resistance and it's inductance so this is roughly one and a half meter long wire which does have resistance and inductance so if I can provide a line that has a smaller impedance then current will go through that rather than through this thing and basically if I take a simple wire connect it to the protected area of the Osler scope and then also connect it to the ground of the power supply miraculously most of the noise has disappeared so what I did here was create a short line that has small impedance and it's also a very thick line so it has a very small resistance and now most of the current rather than going through the otoscope probe ground goes through this line and this in theory should not affect your measurements let's just see if that is true so what my circuit is doing is simply switching a load on and off and I'm measuring the supply of voltage going into the circuit so there's a bit of a voltage drop on it it's perfectly normal but we also have this very nice and very annoying high frequency noise going on now when I take my short ground connection and link it to my austere scope well most of the high frequency noise has gone away but my signal has also been affected so this is not what the signal should look like the signal should look like this so by doing this I didn't just remove the noise I also ruined my measurement now the reason why the circuit is affected can be understood through this diagram so what I got here is a more simplified schematic of the circuit in which my power supply is only the common mode noise source and the supply ground I've modeled my supply ground impedance which are basically the line going from the power supply to the circuit so these have impedance and I got my test circuit creating my differential mode signal which I'm measuring using the oscilloscope this is connected to my channel input and then I have my probe ground impedance which connects again to my test circuit and to the otoscope ground now my added wire is this shorting impedance and when I connect this to the ground on the power supply side basically I'm short-circuiting this pair of wires and depending on how these three components have their impedance current will flow in different directions so we can have two main problems now the common mode noise will be flowing through the shorting impedance since this is much smaller than the probe impedance but if the supply ground impedance is comparable in size to the sum of these two impedances then you will end up supplying your circuit through the shorting impedance and through the program which you do not want and will lead to altered measurements or on the other hand if the supply ground impedance and the shorting impedance are comparable to the probe impedance then your measurement current path will go through these two so both ways you end up having problems now the way around this is to take this ground connection and rather than put it on the power supply side put it directly on the circuit side where your otoscope probe is connected and if I do this then we can see that noise is reduced so the noise reduction effect does appear my signal is no longer affected so the signal looks just the same with or without this connection but this is not always an easy thing to do so to get your ground connection in parallel with your otoscope probe will sometimes work if you have only one probe but if you have multiple probes then this might be an issue so we need a better way of doing this filtration method now the thing to keep in mind here is that common mode noise is a purely AC phenomenon there's no DC component to it and the problem we're having with our simple piece of wire is that this doesn't just conduct the AC component but it also conducts DC so the way to only pass common mode noise through this and eliminate the DC current that's supplying the circuit is to interrupt this and add a capacitor so what I did here is build another piece of wire but I also added a capacitor in the middle so here it's a 4.7 micro farad capacitor and an extra resistor will also help so what I did here was put a 10 kilo ohm resistor and the reason for this is to prevent any sort of large voltage buildup in between the two devices when the probes are not connected so we can put a tank kilohm resistor or even larger doesn't really matter just have a resistor there now if I take this line and connect it to the ground of the otoscope and then also connect it to the ground of the power supply I get both the filtering effect but also the signal is no longer affected so finally by adding the capacitor to the mix I prevented any sort of DC current going through the shorting impedance only AC currents so the test circuit will no longer get supplied from the power supply through this but when we talk about measuring the signal so our differential signal that goes into the otoscope to get measured again one of the advantages of doing this circuit is that the loop created between the signal line and this extra external line has such a large inductance that its final impedance will be much larger than the impedance of the probe ground so this method can work in certain situations to reduce your noise but just make sure that it's not affecting your measurement quality so make sure that your measurement is unaffected by adding this extra wire loop or better yet get the proper power supply that doesn't have so much noise so all in all hope you got some misinformation after this leave your thoughts in a comment thank you for watching make sure to subscribe to be up to date at all my latest videos and see you next time bye bye [Music]

Channel: FesZ Electronics

Views: 37,980

Keywords: fesz, electronics, diy, hobby, electronics tutorial, tutorial, oscilloscope, measurement, noise, common mode, common mode noise, filter, power supply

Id: k-vbFcoCZ_k

Length: 10min 52sec (652 seconds)

Published: Thu May 07 2020