[volt-nuts] How can I make a 2000 V DC meter with an input resistance of at least 100 T ohms?

[kc9ieq]

How about using (or building) an additional 2kV power supply and a sensitive meter movement like a differential voltmeter, adjusting for/measuring the null?  Impedance at null will be theoretically infinate, current will be theoretically zero, and you can measure/monitor the voltage of your second supply directly with the probe/meter of your choice.  
Regards,Chris 


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-------- Original message --------From: "Dr. David Kirkby" <drkirkby at kirkbymicrowave.co.uk> Date: 3/22/18  7:33 PM  (GMT-06:00) To: Discussion of precise voltage measurement <volt-nuts at febo.com> Subject: [volt-nuts] How can I make a 2000 V DC meter with an input
  resistance of at least 100 T ohms? 
I want to measure a high voltage, but put minimal load on the circuit.
Looking at the Keithley electrometers, the input resistance is at least 200
T ohms, but they tend to have a maximum of 200 V FSD.

A 2000 V source, and a 200 T ohm resistor gives a current of 10 pA, which
itself is easy to measure. But one can't buy 200 T ohm resistors. I looked
at RS in the UK, and the highest value resistor I could find is 1 T ohm,
and they are £163 each (around $200).

Maybe fabricating ones own resistor is possible, but I suspect there's a
better way. Keithely manage to keep a 200  T ohm resistance on the 200 mV
range, and there's no way that can be measured with an ammeter, which would
require an ammeter with a full scale deflection of 1 fA, which is much
smaller than the 2 pA FSD on its most sensitive range.

Any thoughts?

Dr David Kirkby Ph.D C.Eng MIET
Kirkby Microwave Ltd
Registered office: Stokes Hall Lodge, Burnham Rd, Althorne, CHELMSFORD,
Essex, CM3 6DT, United Kingdom.
Registered in England and Wales as company number 08914892
http://www.kirkbymicrowave.co.uk/
Tel 01621-680100 / +44 1621-680100
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