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So we're measuring waveforms across certain terminals of an IGBT and noticed that when we extend cables of our differential probes (by about 1.2 meters/4 feet), ringing occurs and we are worried it might possibly destroy the device we are testing.

When we disconnect the said extensions, the length of the cables of the diff probe is about 60cm, and when connected to our test setup, no such ringing is seen.

Did we unintentionally create a tank circuit thru the inductance of the extension cables?

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  • \$\begingroup\$ Yes, the length matters. Yes you are possibly measuring something that is not happening in the circuit. But it is certainly possible for probes to impair signal integrity (or improve it) when connected to a circuit. So there is a chance that connecting the probe could blow up a component. The best course of action depends very much on details. If you can put a high impedance amplified probe right near the net you are probing, that is the best case. From the amplifier back to the oscilloscope will be 50 Ohm cable, with the oscilloscope set for 50 Ohm internal termination. \$\endgroup\$
    – mkeith
    May 26 at 4:01
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that is correct... about 1uh per wire for 1.2m

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  • \$\begingroup\$ Can you expound on how this ringing occurs? Is it related to topics dealing with ground lead impedance? \$\endgroup\$ May 26 at 2:04
  • \$\begingroup\$ it's the resonance of probe pF and lead inductance. That would imply a resonance of about 50MHz for 1pF \$\endgroup\$ May 26 at 2:08
  • \$\begingroup\$ if not then you are seeing mutual coupling from radiated spectrum \$\endgroup\$ May 26 at 2:14

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