LOW side MOSFET turn on

In the image:

  • Channel 1: Drain to source voltage
  • Channel 2: gate to source voltage
  • Channel 3: Switch current
  • Channel 4: Load current

The image shows Vds of low side MOSFET at turn on measured with a passive and differential probe seperately. As you can see, differential probe has a skew compared to passive probe, which is understandable.

But: Why does the reading, after turning on the passive probe, become negative whereas the differential probe still shows the correct reading?

  • 1
    \$\begingroup\$ Hi! didn't you notice your Channel list was illegible? Next time, please take your own time to make your own question readable. \$\endgroup\$ – Marcus Müller Jan 9 '18 at 13:26
  • \$\begingroup\$ How did you adjust the vertical position in the first place? If you used the active probe, and it had a DC offset, the passive probe would appear to have the opposite DC offset (unless you readjusted). Also, the passive appears to have higher gain and we're looking at a short timescale. So ... did you remember to calibrate the passive probe first? \$\endgroup\$ – Brian Drummond Jan 9 '18 at 13:35
  • \$\begingroup\$ @ Brian Drummond i have not done any vertical scaling. The differential probe I used is pico TA058. So can you explain what do u mean by a DC offset. and I did not calibrate the passive probe first. It was already being used in lab by others, So i thought, it must have been calibrated \$\endgroup\$ – Autobot Jan 9 '18 at 13:50
  • \$\begingroup\$ @Autobot : "I did not calibrate the passive probe first. It was already being used in lab by others" this is bad behavior, recalibrate when you start your tests (and write about it in your report) as a lab manager, i've seen day's worth of work thrashed because of that behavior \$\endgroup\$ – Sclrx Jan 9 '18 at 13:55
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    \$\begingroup\$ @Sclrx I checked the calibration again and it was fine. \$\endgroup\$ – Autobot Jan 9 '18 at 15:08

It's probably an effect of parasitic impedance in the passive probe system that doesn't show up for the higher-performing differential probe. It's a pretty minimal negative value, so I wouldn't read too much into it. I'm sure if you give our support center a call an AE would be happy to talk with you in more detail.

  • \$\begingroup\$ What do you exactly mean by higher performing? CMRR? My differential probe is 50 MHz and passive probe is 400 MHz. In this experiment, I am calculating energy losses, and differential probe being of lower bandwidth attenuates my high frequency components, and giving me slower waveforms, which in turn causes my energy losses to be higher \$\endgroup\$ – Autobot Jan 18 '18 at 20:05
  • \$\begingroup\$ I was referring more to probe loading, typically differential probes work better in this regard. What's the frequency component of your edge? \$\endgroup\$ – Daniel Bogdanoff - Keysight Jan 18 '18 at 22:21
  • \$\begingroup\$ @Daniel..As per MOSFET's datasheet rise time, I need to measure close to 20KHz..Input capacitance of my passive probe is <6pF and for my differential probe <7pF \$\endgroup\$ – Autobot Jan 18 '18 at 22:34
  • \$\begingroup\$ sorry, not 20KHz but 20MHz \$\endgroup\$ – Autobot Jan 18 '18 at 22:39

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