Two-signal common mode spike suppression for scope

John Duffy wrote 2 days ago • 0 points

I'm working on a test setup where I've got a problem with a common mode spike.  It's a high voltage (8kv!) capacitor bank discharging in about a microsecond, so just the parasitic capacitance to ground (I assume), causes enough dV/dT to disrupt measurements. I'm measuring voltage and current with a 1000:1 divider and a shunt, respectively, with coax running to an oscilloscope.  Those signals are ~ a couple V once on the coax.  The power supply for the bank is floating, so theoretically there should be no ground current on the coax at all, but even with a shorted input to the coax, I'm seeing a couple volt spike on the scope. I can't seem to post a schematic of the setup here, I'll see if I can in a comment.  The coax/scope ground is connected to one terminal of the capacitor bank, and the divider/shunt are referenced to the same ground. A separate, 16awg ground lead made no difference, I'm guessing any wire length has too much inductance given the ~10kV/uS but I don't see a practical way to get a low-z ground between the scope and test jig (few feet, for safety given the voltages).  

I've tried wrapping the coax around a ferrite core to make a CM choke and it maybe gave me a few dB, but I need like 20dB reduction to get acceptable snr.

Since there's 2 signals, would two separate CM chokes work, or would the ground current not distribute in a way to cancel? Or could I put three windings on one core (one ground going one way, two signals going the other)?  Or is there a better way to do this? I don't want to build a bunch of active circuitry to do fully differential measurements or buy a couple hundred dollar probe.  

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