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| Need help: spikes in current measurement signal conditioning circuitry |
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| StillTrying:
"This all means that the PCB has a wrong design, as there is a big loop around the filtering capacitors:" Not necessarily. I don't understand the differential GND going to the metering IC, I don't think it does anything useful. So what's the plan for measuring the noise spikes, that mostly don't exist. :) For a difficult case such as noise spikes across a component, I always use 2 scope channels. One probe connected across the component and the other probe shorted to the GND as near to the component and other probe as possible, and with both scope probes following exactly the same straight path to the scope. And then compare the difference between the 2 traces. Both scope probes have to be well matched at HF, but you can check that by having them both shorted to GND, or both across the component, where both traces should be identical. |
| balage:
I have tried the measurements with two probes, as you've mentioned. One of them is shorted while the other one is connected to the capacitor. I don't know why, but the transients are displayed/measured with opposite polarity. The situation seems like "noise spikes, that mostly don't exist." The interesting thing is that the power line filter has not helped at all. The load and the device were plugged to the same wall socket, while the power was fed to the PCB through the filter. The transients are the same. There were another trial to measure: I've prepared a triaxial cable that had it outer shield connected to the power line's ground while the inner shield and center conductor were connected to the capacitor. Nothing. The transients seemed to be the same. :D It is not so easy. Still I cannot judge if those transients are there or not. Or if so, than how big are they. |
| duak:
Balage, I am not familiar with the oscilloscope you are using - is it a Rohde & Schwarz? Is one of the channels inverted? Can you display the sum of the two input channels? When I took at the two waveforms and take their instantaneous sums, I think most of the noise will be eliminated. It will not be perfect if the sum is performed after digitization in the oscilloscope. In this case, common mode cancellation is better performed by analog electronics. I also do not understand the measurement scales and your exact test layout. The oscilloscope indicates a 141 Vp-p amplitude? Would you provide a simple connection diagram of your measurement setup including power supply connections to everything connected to the circuit under test? Were you able to get some ferrite cores to place on the coaxial cables? This reduces the common mode transient current before it enters the oscilloscope. If you only have one ferrite core, run both cable though it. If you have two cores, try various combinations - two cables through both cores, one cable through each core and so on. Also try a core on the power cable to the oscilloscope because the transient may also be present on the mains connection. |
| StillTrying:
I think the shorted to GND CH2 helps show that nearly all of the 80V spikes aren't real. But I don't know why the induced spike noise on that CH is showing as just one polarity, it should be impossible. "I don't know why, but the transients are displayed/measured with opposite polarity." I don't know why either!, are you sure that one channel wasn't inverted. When it comes to 22ns wide pulses, the 22nF and 220pF's should mean that everything is shorted together there, and both channels identical. It's a mystery. :) |
| balage:
Yes, it is a Rohde RTH, the 4-channeled version. It is great that is has isolated ones and working from a battery. Maybe the reason for the signal being with one polarity is that the GND of the channels is the GND of the whole circuitry. I have double checked, the channels are not inverted. And here is the most annoying thing: when I change the cables of CH2 and CH3, the signals look the same!! Also, if I swap the wires of one of the coaxes, the signal looks the same. So I think it is coming not from the PCB but induced to the cable or I don't know... I cannot answer all this. Scaling is tricky as well. I have realised that it produces the same 2-3DIV high spikes in the 5V/DIV range and in 40V/DIV range as well. :scared: The scope itself is calibrated and working good. It seems not the scope nor the circuit is bad, but the designer should be fired. Jokes aside, I will talk about this above with somebody from Rohde to investigate this paranormal measuring thing. --- Quote from: StillTrying on July 27, 2019, 01:36:03 pm ---It's a mystery. :) --- End quote --- When something like this happens, I always regret I've not paid enough attention at the university... The ferrite rings have not helped, everything looks the same. When both coaxes were shorted and soldered to the GND of the circuit, it was producing the same spikes. So the transients are not there. I am stepping forward and will continue the field tests. |
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