Electrometers and Picoammeters

For the design and construction details of ion guns, necessary for more advanced designs and lower vacuums.
ian_krase
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Re: Electrometers and Picoammeters

Post by ian_krase » Thu Jan 04, 2018 8:55 am

I'd be curious about suggestions on how to use 5886 tubes in an electrometer frontend suitable for use with the Keithley 610/616 range switch, beyond just cribbing from the Keithley manual. (Would be nice to make the rest of the circuit with op amps, for example, rather than endless carefully matched bipolars and passives everywhere.)


It's a shame we can't have a run of new production 5886s made.

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Richard Hull
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Re: Electrometers and Picoammeters

Post by Richard Hull » Thu Jan 04, 2018 4:33 pm

I believe that only the input FETs in the older solid state keithleys are matched. Even the 5886's were often sold matched. All front ends of electrometers rely on matched input devices.

However, modern IC's with dual, fempto-amp bias, OP Amps in them are very well matched and the need for older matched items is going away.

The LMC 6082, 10 fempto-amp bias, dual op amp is a very inexpensive example of a great starting point front end for a homemade electrometer.

I used the LMC 6081 single op amp as a cheap and dirty front end in a lot of electrostatic measurement systems using a 50 gigohm input resistor which ruined its low end sensitivity, lowering it to the pico-amp range. Hype sensitive electrostatic field and even magnetic field detectors can be made with this little 8 pin IC. IC's of this ilk are used with 100meg bias resistors in smoke detectors.

Insulation and isolation are the big bugaboos in all electrometer circuits.

Cool story: I obtained a glass vacuum capacitor of 50pf. I cleaned the glass with absolute alcohol. I dried it using a hair dryer. I placed it in copper enclosure where the capacitor was suspended by a thread super glued to the center of the glass body. A tiny circuit board contained a unity gain 6081 with no bias resistor on the positive input of the op amp. This " lifted + leg" of the IC was connected to the capacitor. I charged the 50pf with a 1.5 volt AA cell and immediately removed it. The op amp read 1.55 volts and after a week or, the capacitor's continuous in circuit drain showed a loss of only .08 volt.

The current LMC6081 is only $2.59 from Jameco

https://www.jameco.com/webapp/wcs/store ... llDownView

click on data sheet for more info.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

ian_krase
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Re: Electrometers and Picoammeters

Post by ian_krase » Thu Jan 04, 2018 8:15 pm

There definitely are plenty of Electrometer grade opamps. I was asking how you get that invulnerabile vacuum tube goodness.

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Re: Electrometers and Picoammeters

Post by John Myers » Thu Jan 04, 2018 9:52 pm

Would be nice to make the rest of the circuit with op amps, for example, rather than endless carefully matched bipolars and passives everywhere.
You can use an op-amp or two to replace the analog backend since op-amp are just a packaged version of all that circuitry.

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Richard Hull
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Re: Electrometers and Picoammeters

Post by Richard Hull » Fri Jan 05, 2018 3:31 am

It is far too easy to make your own modern op amp electrometer if you are careful regarding insulation, shielding, etc. I once gutted a Keithley 600 electrometer and left only the switch network and resistors in the box substituting an LMC6082 and LM324 follower op amps to work the meter. A bit more troble than it was worth, considering all the other keithley's I had in storage.
5886 electrometer tubes are easy to locate. I have them for sale. Tube electrometers love to be turned on and once balanced an aligned, left on forever.

I think I also have a 610A or 610B for sale.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

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