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capacitive voltage divider measurement problem,

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Joined: September 8th, 2015, 5:53 am
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capacitive voltage divider measurement problem,

Post by mehran » September 23rd, 2015, 11:35 am

In our LAB we have keithley 6517B. And I am trying to measure the voltage across(C2) the LOW VOLTAGE ARM of capacitive divider. My capacitance values are C1= 0.050PF and C2 = 25PF. And I am using DC voltage source V=1000V. I know that the keithley has its input capacitance = 20PF. And I am using TRIAX cable(with banana connectors at the end) . Which has its capacitance around 130PF for 1 meter.So, I think the total LOW VOLTAGE ARM capacitance , i mean C2 will become C2 = 25PF+20PF+130PF= 175PF roughly. As I am doing UNGUARDED measurement. I have connected the RED of the Triax to HI of my circuit. And Black of triax to LOW of my circuit and I have also grounded the LOW of my circuit.I seemed two cases:

1. I have connected the TRIAX cable to my circuit through small extention wires. And I measure the voltage across C2 . I found 4.6V (which was going to decrease due to discharge which was obvious AS I have connected 100TOhm resistor parallel to C2) against 1000V DC. While according to the capacitance ratio its value should be around 0.3V.

2. Secondly, Then I thougt may be the problem is with the extensions and I removed the extension and dirctly conneted the triax plug to my circuit.
And I got voltage across C2 which was 0.8V. Moreover It was going to increase rather than decreasing. And after a minute then it goes decreasing.
I have tried several times by using different cables but no effect.
Please, can anyone guide me where am I missing something?

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Re: capacitive voltage divider measurement problem,

Post by brian.d.smith » September 24th, 2015, 11:55 am

If your capacitors and high value resistors are unguarded and unshielded, there will be induced noise from the electrical environment and inducted charge from proximity to people.

In general, any resistance above 1G should be shielded and guarded.

The capacitor that is 0.05pF is going to be difficult to isolate due to stray capacitances nearby.

It would be helpful to have you post some images of the devices to be measured and the cables, connectors, etc.

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