Removing 60 cycle noise from oscilloscope when measuring current via shunt - Page 1
Forum section: electronics / beginners

author: javadesigner
author_rank: Contributor
timestamp: March 08, 2021, 12:59:02 am
content: Hi all:I have a a R&S RTB2004 Oscilloscope but this question probably applies to any oscilloscope.I am trying to measure a small amount of current via a shunt resistor. The amount of current to be meaured is between 0-200 uA ).I see a noise of 60 cycles and a voltage fluctuation of +/- 20mV (which adds 100 uA current noise)This noise and induced current is overwhelming my signal, my triggers and my measuring displays. I was able to quieten the trigger somewhat by specifying "ignore HF".But my measurents are still all screwed up. Instead of showing Vpeak-to-peak or Vamplitute correctly, I am seeing Vpp/Va values that fluctuate all over the place, because of this induced +/- 20mA, and the corresponding added current noise (100uA).Is there any way to ignore or cancel out this nduced voltage while measuring current via a shunt resistor ? One end of the probe is attached to one side of the resistor and the probe ground to another side of the resistor, so essentially this is a floating ground, which could be adding to the noise. Not sure how else I could measure current if I don't do it this way but then the noise overwhelms.

author: TimFox
author_rank: Super Contributor
timestamp: March 08, 2021, 01:02:41 am
content: How and what exactly are you connecting as a probe, and what exactly did you do with the ground lead?When you switch the input to “Gnd”, what do you see?

author: javadesigner
author_rank: Contributor
timestamp: March 08, 2021, 01:09:09 am
content: Hi Tim, I updated my post to clarify and answer your question.It is a floating ground - one end of the osc. probe to one end of the shunt and the ground end of the osc. probe to the other end of the resistor.I see the issue in both 1x and 10x probe modes. Not sure what you mean by switching to ground ?

author: WattsThat
author_rank: Frequent Contributor
timestamp: March 08, 2021, 01:22:49 am
content: The vertical input coupling selector. AC-DC-GND. Does the noise go away when the selection is GND?

author: WattsThat
author_rank: Frequent Contributor
timestamp: March 08, 2021, 01:25:47 am
content: What’s the shunt connected to? AC? DC? What the shunt value (resistance)?You do know the ground clip of the probe is connected the ac mains socket earth, yes?

author: javadesigner
author_rank: Contributor
timestamp: March 08, 2021, 02:25:57 am
content: Quote from: TimFox on March 08, 2021, 01:02:41 amQuote from: WattsThat on March 08, 2021, 01:22:49 amOn the RTB-2004, under coupling, I only see AC-DC, not ground.In a separate section (below the coupling section), there is a GND toggle switch. If I toggle that, the display signal is always 0 volts, no noise, regardless of what the test circuit voltage is.Quote from: WattsThat on March 08, 2021, 01:25:47 amThanks for this pointing this out. I may have seen the video but had totally forgotten about that. Thankfully escaped blowing up anything this timeAttached are some diagrams showing (1) the shunt resistor and (2) oscilloscope outputs with noise.The shunt resistor is 10 ohm (I did not have 1 ohm handy) and I set the probe to be in A/current mode, with 10 V/A conversion. My assumption was 10 Volts/10 Ohms = 1 A, so 10 V/A. I've never used a shunt resistor before, am a total n00b so I could be doing this entirely wrong.In the osc. signal pics, I haveenvelopemode turned on, so you can see the voltage variation (the thin yellow lines). Since the voltage measurements fluctuate so much, the current measurements fluctuate a lot as well.

author: penfold
author_rank: Frequent Contributor
timestamp: March 08, 2021, 02:51:32 am
content: If one leg of your shunt resistor is on chassis earth anyway, why bother with that setup?Without a scope probe earth connection to your circuit you have large impedance in your return path for each probe AND using two probes as such has very poor common mode rejection, does connection the probe ground to the chassis ground on the circuit help? Otherwise a differential probe would be in order.

author: srb1954
author_rank: Super Contributor
timestamp: March 08, 2021, 02:52:31 am
content: Quote from: javadesigner on March 08, 2021, 02:25:57 amThe scope timebase is really fast so it is not possible to determine if it is 60Hz noise or some other source. Please provide a trace picture with a 2ms/div timebase setting or slower so we can see an entire 60Hz cycle.

author: WattsThat
author_rank: Frequent Contributor
timestamp: March 08, 2021, 04:42:30 am
content: Not sure what you’re doing but if this just a learning exercise, I’d use the 1K as the shunt, just move it to the other side of the led. Two orders of magnitude improvement in signal quality.The power supply could have 60Hz leakage, likely if it’s a switch mode unit.

author: javadesigner
author_rank: Contributor
timestamp: March 08, 2021, 04:50:12 am
content: Quote from: penfold on March 08, 2021, 02:51:32 amThanks - and sorry - the diagram was a little confusing. I've updated it to clarify I am using only one oscilloscope probe across the shunt, not two probes.

author: javadesigner
author_rank: Contributor
timestamp: March 08, 2021, 04:56:41 am
content: Quote from: srb1954 on March 08, 2021, 02:52:31 amThanks.Attached are some more pics at a higher div timebase (2 ms/div). In these pics the probe and ground are across the shunt resistor.Also attached is a pic at 5 ms/div time base, which show a 60 second cycle (the measurement shows about 16.5 ms per cycle) and you'll see that this is +/- 20 mV.To take this last pic, I left the signal side of the probe disconnected the ground part of the probe entirely (ground not connected to anything)

author: javadesigner
author_rank: Contributor
timestamp: March 08, 2021, 05:01:22 am
content: Quote from: WattsThat on March 08, 2021, 04:42:30 amPurely a learning excerise. Ok, I'll try this.Quote from: WattsThat on March 08, 2021, 04:42:30 amIt is a Siglent SPD 3303X-E power supply. Using Channel 1, V = 2.0, I = 0.2AHowever,if I disconnect the power supply completelyand then view the the oscilloscope probe connected across the shunt resistor, I still see the samebehavior. 1 mV extra on either side of the signal, which should be a flat 0 volts exactly. I'm not near any fluorescent lights.

author: Tom45
author_rank: Frequent Contributor
timestamp: March 08, 2021, 05:09:29 am
content: Is the circuit diagram you showed, the entire circuit?What does the scope show if you put the scope probe across the power supply output? Best to use AC coupling so that you can better see any noise.Do you have the negative terminal of the power supply tied to the power supply's green ground terminal?

author: WattsThat
author_rank: Frequent Contributor
timestamp: March 08, 2021, 05:15:51 am
content: Third trace: The 60 Hz noise is just that. Stray mains noise picked up by an unterminated connection. Meaningless measurement.The first two traces are noise from a switch mode power supply. Decrease the horizontal time base until you can see discrete pulses and that will be the switching frequency of the power supply.Check the specs of your power supply. 1mv of noise would look like what?https://siglentna.com/wp-content/uploads/2020/10/SPD3303X_DataSheet.pdfHint: Start with the basics. Put a 1k resistor across the supply, set the output at 2V. Connect the scope across the resistor. Set the scope to AC coupling. What do you see?

author: javadesigner
author_rank: Contributor
timestamp: March 08, 2021, 06:07:34 am
content: Quote from: WattsThat on March 08, 2021, 05:15:51 amThanks again for the informative reply/suggestions.I've removed all the shunt crap and made a simple circuit with only a single 1K resistor.Also set the scope to AC coupling. As you can see from the attached screenshot- Vpp is about 80 mV, not 0 volts, essentially 40mV of noise on either side of the signal.However - here is the interesting thing - I see the exact same trace/behavior on the scope (approx 80mV of noise)when I switch off the power supply entirely.(ie hit the off button on the siglent).

author: tautech
author_rank: Super Contributor
timestamp: March 08, 2021, 06:19:31 am
content: What happened to the 1mV/div scope setting you've used until now ?

author: javadesigner
author_rank: Contributor
timestamp: March 08, 2021, 06:23:29 am
content: Quote from: tautech on March 08, 2021, 06:19:31 amSorry, I tried using that but that clips the signal. I've kept the 2 Volts and the 1K resistor values the same as before - as suggested by WattsThat above: "Put a 1k resistor across the supply, set the output at 2V. Connect the scope across the resistor. Set the scope to AC coupling".So the only way to see the signal is to boost the mv/div. Would you like me to use a different (larger resistor) instead ?

author: tautech
author_rank: Super Contributor
timestamp: March 08, 2021, 06:32:44 am
content: Quote from: javadesigner on March 08, 2021, 06:23:29 amYeah but that screenshot was with the PSU OFF, right ?With 0V into the circuit can you still see 60 Hz ripple at 1mV/div ?Eliminate to investigate.

author: javadesigner
author_rank: Contributor
timestamp: March 08, 2021, 07:16:52 am
content: Quote from: tautech on March 08, 2021, 06:32:44 amHi - that screenshot was with the PSU ON. I just mentioned that I saw similar with PSU off but didn't include a screenshot for the latter.I've added 4 more screenshots below.For this experiment - The PSU is off and to be doubly sure, also unplugged from the power outlet throughout.1) The first two are for AC and DC coupled modes with the PSU off and the PSU connection pin also removed from the breadboard circuit. Quite a lot of noise. Notsure what the frequency is, since measured frequency is all over the place (not like the 60hz measured before with ungrounded probe)2) Second two are AC and DC coupled modes with the PSU off (same as above). This time however, added an empty wire to the power rail on the breadboard. (this wire isn't connected to anything, just a additional empty wire into the power rail of the breadboard). I see way more noise this way have to dial upto 5mv/div to see the whole thing.

author: tautech
author_rank: Super Contributor
timestamp: March 08, 2021, 07:37:19 am
content: OK, good. Pretty obvious there's no 60 Hz in that however there is some HF noise there and possible some SMPS nearby being picked up by the probes......not at all uncommon.Start increasing the timebase to where you can hopefully trigger on it and for most SMPS it's likely in the 15-20 KHz range.Some scope users are so afflicted by RFI they must do sensitive measurements in a tin box (Faraday cage) however just moving away from the bench to another location can prove if it's RFI or not but don't discount your bench lighting isn't the cause.Just because the modern DSO can do sensitive measurements doesn't make dealing with all the problems of them any easier.....especially down in the uV !

author: Stray Electron
author_rank: Super Contributor
timestamp: March 09, 2021, 12:35:32 am
content: Some LED and CF room lights are insanely noisy. Try switching off different lights in that room and the surrounding area and see if the noise goes away or at least greatly diminishes.Also, I've never used a Siglent PS but I've used other cheap Chinese made ones and all of the ones that I tested all generated gobs of RF noisy so I'msurprisedthat you're not seeing a change in the noise level when you turn off your PS. If you have any other Chinsesium power supplies in that area turn them off and see if that makes a difference.If you can get your hands on a old linear PS, particularly if you can get a top brand like HP, Harrison or Lambda, try it in place of your switcher PS and see if the high frequency noise goes away.

author: tautech
author_rank: Super Contributor
timestamp: March 09, 2021, 01:07:31 am
content: Stray ElectronThe PSU in question is a 3ch linear PSU with a CV Ripple & Noise ≤300μVrms spec.It won't be the problem.

author: javadesigner
author_rank: Contributor
timestamp: March 09, 2021, 01:27:38 am
content: Quote from: tautech on March 09, 2021, 01:07:31 amRight - its definitely not the power supply.I haven't been able to located the noise source unfortunately. Turned off all nearby lights, there is no computer power adapter sitting nearby, etc or anything else connected to that power outlet. The noise goes up substantially if I connect extra empty wires into that same breadboard so it is picking up high frequency radio interference from somewhere.I ended up using High Res mode to get rid of most of the noise. Thanks to all you posted in this forum - very much appreciated. You guys rock

author: SilverSolder
author_rank: Super Contributor
timestamp: March 09, 2021, 05:15:06 am
content: Quote from: javadesigner on March 08, 2021, 06:07:34 amTry this: Instead of hitting the Off button on the power supply, actually unplug the power supply completely from the outlet. Does the noise disappear now?If so - you are having a Ground Loop experience!

author: tautech
author_rank: Super Contributor
timestamp: March 09, 2021, 06:12:52 am
content: Quote from: SilverSolder on March 09, 2021, 05:15:06 amThe PSU outputs are not mains Gnd referenced.

author: mikerj
author_rank: Super Contributor
timestamp: March 09, 2021, 09:41:00 am
content: Use two channels in subtract mode to perform a differential measurement, this should provide a decent amount of common mode rejection. You could also enable averaging on the scope if the signal you want to view is DC or repetitive in nature.

author: SilverSolder
author_rank: Super Contributor
timestamp: March 09, 2021, 12:49:09 pm
content: Quote from: tautech on March 09, 2021, 06:12:52 amThere can still be capacitive coupling - I've been caught by that!