Products > Test Equipment

Better diff probe to avoid inaccurate scopes

<< < (4/4)

David Hess:

--- Quote from: Weston on March 21, 2023, 10:26:34 pm ---By what mechanism do you think "leakage current" is causing this? The differential probe has a 4M input impedance, if you are probing a low impedance node such as the DC bus voltage it seems unlike that any current though the input impedance of the probe would cause issues.

If the waveform is distorted is it possible that you are exceeding the common mode input voltage of the probe? That would distort the waveform produced by the probe.

If the DUT is floating and has no ground reference the common mode voltage is poorly defined and could reach some high DC potential. Floating the scope would not necessary fix this, no would a better differential probe. You need to provide some sort of ground return for the DUT to limit the voltage it could float to.
--- End quote ---

What you describe could be the problem here.  Differential probes must be used with a ground return to control the common mode voltage.  For instance they cannot be used with an isolated oscilloscope input, or with an isolation transformer on the device under test, without the risk of the common mode leakage causing the common mode voltage to go out of range.  Tektronix specifically mentions this limitation in the documentation for their differential probes or maybe isolated inputs.

gf:
If an ordinary differential probe is not good enough, what about optically isolated probes?
Seems they support common mode voltages of tens of kV, and CMRR of 150dB and more :)
Price is scaring, though :scared:

David Hess:

--- Quote from: gf on March 28, 2023, 11:09:40 pm ---If an ordinary differential probe is not good enough, what about optically isolated probes?
Seems they support common mode voltages of tens of kV, and CMRR of 150dB and more :)
Price is scaring, though :scared:
--- End quote ---

They are a higher performance solution *if* their high common mode capacitance can be tolerated, which is the case if the measurement is made with respect to the emitter or source of the output transistor.

bson:
I'd suggest finding the root cause of the oscillation rather than adjusting your measurement method to make it go away.  If it's so close to oscillation that just putting a probe on it triggers it you have a problem, one which will eventually happen in some production setup.  Be happy you found it now rather than later, and use the probe to reproduce it - and focus on tracking down the root cause.  Then design it out of existence.

Navigation

[0] Message Index

[*] Previous page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod