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Registered Member #1316
Joined: Thu Feb 14 2008, 03:35AM
Location: Cambridge, MA
Posts: 365
Why do you need such a high calibration voltage? 10x probes are calibrated with a 0.5v square wave from a built in generator on the scope. You should be able to get away with a 50 volt calibration signal, or at the very least, 500 volts.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Weston wrote ...
Why do you need such a high calibration voltage? 10x probes are calibrated with a 0.5v square wave from a built in generator on the scope. You should be able to get away with a 50 volt calibration signal, or at the very least, 500 volts.
Im thinking 900 volts may be enough, but Proud Mary and I ran into problems with putting 100 volts in to several hundred megaohm resistances then seeing that the current to function properly, so nano and pico-amps seem to not work so well with DMM's...
Also remember this is a 10,000:1 probe, so vertical amplifier noise matters too...
let me see if i can find those threads...
here is PM's previous work... A 75kV 1000:1 Voltage Divider With 3G Input Impedance
Proud Mary wrote ...
Patrick wrote ...
hey Proud Mary... i got out my old 2 giga ohm, 1000:1 resistive voltage divider and connected it up too a 270vDC (+- 1.2 %)source then used a miliVolt meter to mearsure the low side but it wasnt stable it should have read 27mV but instead it would go down to 9mv and up to 41mv very slowly, no loose leads seemed to be the cause,i have no idea if its become and antenna or not? i need help. any ideas?
With only 135 nA bottlenecking through your 2G @ 270V, you'll get erratic performance alright, Paddy.
Firstly, the the input offset current of the DVM input device will become significant in the nA regime. Then the input of your meter should be considered as Rin parallelled by Cshunt, a combination which has a time constant, as Cshunt takes time to charge.
With only 135nA to be split between Rmeasure, Rin, the offset current and the charging of Cshunt, no wonder it takes time!
If you measure charging time t, then you'll be able to back calculate Cshunt, and see whether the answer looks plausible or not.
Leakage currents and input offset current will be signicant when contrasted with your miserable 135nA, and perhaps induced exogenous noise currents too, I would think, as you suggest, so you're most likely looking at a multifactorial error.
My advice: put an electrometer transconductance amplifier ahead of your DVM, but failing that, put some real voltage across it!
Added Much Later: Pity I didn't think of all this when designing my own voltage divider!
Registered Member #2901
Joined: Thu Jun 03 2010, 01:25PM
Location:
Posts: 837
Would this work as a relatively simple but high speed gate driver (can't get the simulation to run) :
The transformer would be a couple of windings of bifilar HV wire on a toroid, hence the low induction and coupling. It's similar to the one from the patent I linked, but I don't really see the point of the second transformer. AFAICS this would work too.
Registered Member #1316
Joined: Thu Feb 14 2008, 03:35AM
Location: Cambridge, MA
Posts: 365
Whats you input impedance? The tek 1000x probes have an input impedance of 100M Ohms. Will your probe have an input impedance of 1G Ohm? Using 1200v (max voltage for common switchmode IGBTs) You would be getting 1.2 microamps through the probe.
Also, do you need a 100Khz reprate? Or Just a 10 microsecond pulse?
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Weston wrote ...
Whats you input impedance? The tek 1000x probes have an input impedance of 100M Ohms. Will your probe have an input impedance of 1G Ohm? Using 1200v (max voltage for common switchmode IGBTs) You would be getting 1.2 microamps through the probe.
Also, do you need a 100Khz reprate? Or Just a 10 microsecond pulse?
well i guess it could be set to use a couple of pulses, seeing as how the trigger on the scope could be used for hold. anyway my HV probe is 375 Mohms of DC resistance, so im thinking as i said before that 900 volts might be enough.
EDIT: 900 volts / 375 Mohms = 2.4 uAmps. so im thinking 600 to 900 volts ought to be enough.
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