How to get large resistor 1 GOhm?

Analog Ground

Joined Apr 24, 2019
460
Controlling leakage with cleanliness is important but also the use of a "guard ring" on the input to shunt away leakage currents. Just search "op amp guard ring".
 

Deleted member 115935

Joined Dec 31, 1969
0
out of interest,
resistance of "humid air " is what per mm ?
resistance of "sweat" left on a component is ?
 

Thread Starter

LAOADAM

Joined Nov 21, 2018
862
That will be a very interesting device when it i completed. Years ago I studied the plans published for "the femtofarad Meter" as part of a work project. But we needed to measure dielectric absorbtion and its variation and changes with aging and temperature. So measuring capacitance was not enough, nor was measuring leakage current.
So what capacitor characteristic would the system be evaluating?
Thanks.
Its simple for me, I just like to measure the capacitance even no need a high accuracy.

We commonly use 1GΩ encapsulated in glass in the front end of charge-coupled preamps.
Thanks.
Is that means I can find it in some circuit?

I did a lot of measuring in the pA region, In one case, it was better to count Coulombs and divide by time.

Systems were not that difficult to set up, except the first time.
Thanks.
 

BobTPH

Joined Jun 5, 2013
8,813
So what is the circuit? If you think you are going to charge the capacitor through your 1G resistor while measuring the voltage, you would need a measuring device with at least 10G input impedance.

Bob
 

MrChips

Joined Oct 2, 2009
30,712
What's wrong with buying it from your usual supplier for $1 each?
If you are trying to measure capacitance, what range of capacitance values do you want to measure?
What is your method for measuring capacitance?
 

Thread Starter

LAOADAM

Joined Nov 21, 2018
862
What's wrong with buying it from your usual supplier for $1 each?
If you are trying to measure capacitance, what range of capacitance values do you want to measure?
What is your method for measuring capacitance?
Thanks.
I didn't find locally, takes too much time online.
 

MrChips

Joined Oct 2, 2009
30,712
I am going to assume that you are attempting to measure the capacitance value by measuring the time it takes to charge the capacitor to a given threshold voltage.

The reason that your method calls for a very large resistance value is because you want to measure very small capacitances, lower than 10pF?

There is another way to measure small capacitance values.
Instead of measuring period, measure frequency.
Place the unknown capacitor in an oscillator circuit and measure the frequency of oscillation.
 

Thread Starter

LAOADAM

Joined Nov 21, 2018
862
I am going to assume that you are attempting to measure the capacitance value by measuring the time it takes to charge the capacitor to a given threshold voltage.

The reason that your method calls for a very large resistance value is because you want to measure very small capacitances, lower than 10pF?

There is another way to measure small capacitance values.
Instead of measuring period, measure frequency.
Place the unknown capacitor in an oscillator circuit and measure the frequency of oscillation.
Thanks.
I don't really have a measurement range yet, on planning.
 
@MrChips I think the pre-amp crapped out on an XRD detector. The only thing EDAX had at the time was a PDP-11 Q-bus card, so I put it in a box by itself and changed the gain.

Used a stand-alone MCA. We did eventually upgrade to a nice Amray electron microscope with an EDAX XRD with the bells and whistles under a maintenance contract.
 
1GΩ glass encapsulated resistors are mounted on Teflon standoffs to avoid stray leakage currents.
I know the glass resistors well. I did an I-V converter design that I used an OP-41. It was the 8 lead TO something package, leads in a circle. I put the one input pin on a PTFE standoff. My ranges for +-10V out were 100, 10, 1 and 0.1mA. I used mercury wetted relays. Since this was designed to be a 4-terminal/2-terminal I-V converter with bias, the measuring leads had 400 M-ohm resistors to ground as bias current bleeders.

What I did not do, wasn't allowed to do was to deal with Vos. There was no way to null Vos.
We only really cared about AC performance, so 40pA of offset was no big deal.

What I could not do was output ZERO Volts. I was going to solve for Vos and null it.
 
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