High value resistors, gigaohm to teraohm.

  • Thread starter Thread starter Dmytry
  • Start date Start date
  • Tags Tags
    Resistors Value
Click For Summary

Discussion Overview

The discussion revolves around the challenges of creating high-value resistors in the gigaohm to teraohm range, particularly for use in sensitive ionization chamber projects. Participants explore various methods, materials, and circuit designs to achieve stable and effective resistance for measuring extremely low currents.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • One participant experiments with pencil traces on ceramics to create high-value resistors, seeking historical methods for stability.
  • Another suggests that if resistance is not critical, the reverse leakage of a diode might suffice as a bleeder, though stability is questioned.
  • Concerns are raised about stray capacitance and the need for linearity in resistance measurements, with a potential shift to using reed switches for discharging inputs into an op-amp.
  • A proposal is made to use multiple high-value resistors in series to mitigate contamination and leakage issues.
  • One participant references a website recommending an integrator to eliminate the need for a resistor, emphasizing the requirement for op-amps with very low input bias currents.
  • Another participant points out mathematical inaccuracies in the referenced article regarding capacitor charging and voltage calculations.
  • Specific op-amps suitable for low bias currents are mentioned, including the LMC660, LMC 6044, and LMP7721, which is designed for ionization chambers.
  • There is a discussion about integrating on the internal capacitance of the ionization chamber versus using an external capacitor, with considerations of voltage rise times and background current levels.

Areas of Agreement / Disagreement

Participants express various approaches and ideas, but there is no consensus on a single method or solution. Multiple competing views and suggestions remain throughout the discussion.

Contextual Notes

Participants note limitations related to contamination, stray capacitance, and the need for precise specifications in their designs. There are unresolved mathematical steps regarding voltage calculations and the implications of using different circuit components.

Dmytry
Messages
510
Reaction score
1
Is there some good way to make 109 to 1012 ohm range resistors? (buying them is expensive).
I've been experimenting with pencil traces on paper, that is understandably unstable, going to try with pencil traces on the ceramics then cover it with glue. I was wondering if there's some well known way to do it, from the good old days.

I need those resistors for sensitive ionisation chamber project (measuring currents of several fA. 1000 cubic centimetre air ionisation chamber has current of approximately 1E-8 A at dose rate 1 Sv/h , i.e. 1fA at 0.1 mkSv/h .). As an alternative I was thinking to use reed switch relay to discharge.
 
Last edited:
Engineering news on Phys.org
If the actual resistance is not critical and just needs to be a bleeder then you may find that the reverse leakage of a diode is enough to do it. I suspect you want something stable though with a known spec'd resistance.
 
Averagesupernova said:
If the actual resistance is not critical and just needs to be a bleeder then you may find that the reverse leakage of a diode is enough to do it. I suspect you want something stable though with a known spec'd resistance.
I can measure it no problem, just need it to be linear and not have a lot of stray capacitance and not to work as voltage source (liquid in capillary tube wouldn't work well).
I may end up building some altogether different design, using reed switch to discharge the input into op-amp periodically. The reason I want to go with 'normal' amplifier is that this way i can perhaps detect individual tracks.
 
I guess I would suggest multiple mustiple high value resistors in series. Any contamination can cause significant leakage.
 
Interesting problem. This website recommends using an integrator which would remove the need for the resistor.
http://www.tmworld.com/article/319701-Femtoamp_fA_measurements.php

You do need a opamp with less than a fA input bias current though. But I guess you needed that anyway to pick the voltage off the resistor.

I've heard of amps with 10s of fA input bias for light sensing circuits. Not sure about less than 1 fA. Maybe you can do a manual trim on a 10fA amp or something...
 
Last edited by a moderator:
es1 said:
Interesting problem. This website recommends using an integrator which would remove the need for the resistor.
http://www.tmworld.com/article/319701-Femtoamp_fA_measurements.php
There's some problems with math in that article... e.g. 1fA , 100 seconds, 100uF cap, you will get 1 nanovolt, not 100 microvolt. should use 1 nF and 100 seconds to get 100 microvolts, of course.
You do need a opamp with less than a fA input bias current though. But I guess you needed that anyway to pick the voltage off the resistor.

I've heard of amps with 10s of fA input bias for light sensing circuits. Not sure about less than 1 fA. Maybe you can do a manual trim on a 10fA amp or something...
I found the less than 1fA opamps... LMC660 , LMC 6044 , and the best one (made for ionisation chambers) is LMP7721 .

My other idea is to integrate on the internal capacitance of the ionisation chamber, then discharge through opamp using a reed switch, getting a voltage pulse. Voltage pulse would be a lot easier to amplify, I can remove low frequencies. And this way i can use high bias current op amp.

Also this guy built real good chambers using just a FET:
http://www.techlib.com/science/ion.html
 
Last edited by a moderator:
Just out of curiosity, what's the advantage of using the internal capacitance of the chamber over just a cap?

That is an awesome link. Lot's of good ideas there. The guy's a really good circuit designer. I would definitely copy if I were to build a chamber. :)
 
es1 said:
Just out of curiosity, what's the advantage of using the internal capacitance of the chamber over just a cap?

That is an awesome link. Lot's of good ideas there. The guy's a really good circuit designer. I would definitely copy if I were to build a chamber. :)
Well, faster voltage rise. Capacitances are small... if I have current of 1fA, then 1pF takes 1000 seconds to charge to 1v .
He says tho he get 40fA at background which is interesting. Maybe he got a lot of radon.