How to build a 1E+11 ohm resistor?

[steve_rb]

I need a 1x10^11 OHM resistor for a very sensitive amplifier. Anyone knows how to build one ? A friend was proposing to use 4 layer of plastic

 

[FvM]

100 GOhm resistors are made from a number of manufacturers, small chips upto ultra stable glass sealed types. Some available of stock from distributors like Digikey. I don't believe that a selfmade resistor will meet the target value better than a factor of 10 to 100 lower or higher.

https://riedon.com/media/pdf/HVS.pdf
https://www.ohmite.com/cat/res_maximox.pdf
https://www.ohmite.com/cat/res_rx1m.pdf

 

[steve_rb]

My amplifier is inside hi vacuum and is connected to a Faraday cup collecting ions. Mentioned resistor is an ultra stable glass sealed one with a capacitor in parallel with it (attached photo). After about 1 year not being used and with air inside the amplifier it seems its resistance is dropped because its sensitivity to small signals are lost. Still it responses to the finger when I get my finger close to the input of the amplifier. It was working ok a year ago. Is it possible that the capacitor has problem or the resistor?

 

[FvM]

In my view, resistor failure is most unlikely. The resistor will even withstand kV surges, which can't be assumed for the capacitor and for the amplifier still less. You can measure the resistor by applying a voltage and measure the current with a usual digital multimeter in voltage range. E.g. 200 mV range, 10 MOhm input resistance has at least 0.1 nA resolution.

Knowing the appliclation, it seems almost ridiculous to consider a self-elaborated resistor, I think.

 

[steve_rb]

I have attached the amplifier circuit here. I did changed the OP amp and ICH8500 but still the same. Amplifier responds to my finger movements and I see printer printing something when I move my finger in about 2 cm from input pin but it should start printing the 100G ohm noise even without finger movement. What do you think might be the problem?

 

[FvM]

but it should start printing the 100G ohm noise even without finger movement.

Not necessarily, depends on the capacitor value, too. In any case, I would measure the actual resistance of R and C, as suggested.

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P.S.: You can also check the range of C and R by applying known charge amount to the input (voltage step through small capacitor) and observing the output voltage step and decay time constant.l

 

[steve_rb]

Yes it should because commissioning documents indicate with 10^11 ohm resistor recorder should print the noise even without any ion (< 0.4 mV noise on the recorder chart). But now all I see is a straight line. Also service manual says you should see noise first and then the ion peak whenever they reached the Faraday cup.