Charge amplifiers

[zahid61]

Hi dear all,
Designing a charge amplifier, what primary parameter one should look for? I mean if one has to amplify presence of a few ions?
Thanks
Zahid

 

[Skogsgurra]

Insulation, capacitance, shielding, guards.

A few ions, that's sub-attocoulomb. Can it be done?

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[MacGyverS2000]

Are you designing this with COTS components or at the silicon level?


Dan - Owner

http://www.Hi-TecDesigns.com

 

[logbook]

I would think this requires an amplifier based on a vacuum tube cascading device, sort of like either a geiger tube or scintillation counter. You are not using an opamp or any other semiconductor device because the noise would be too high. Geiger counters take one disintegration and avalanche it up to a big number of electrons. This is "amplification", but requires a physicist rather than an electronics designer.

 

[jimkirk]

http://www.edn.com/contents/images/6434367.pdf

This article has some interesting info on measuring sub-nano-amp currents. It has some good info on the pitfalls of measuring very small (femtoamp) currents, one of which is when using a capacitor to integrate currents for measuring. Physically large caps are more prone to interactions with cosmic rays, (read: ionization events). Since your application is to measure (or maybe count, since you're looking at basic chunks of charge) ions, you might turn this problem to your benefit.

A plug for

http://www.rfcafe.com

which fortuitously had this link.

 

[itsmoked]

I was just looking at some op amps that had sub 10pA input current requirements. If you're interested. Linear had them I think.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Skogsgurra]

"A couple of ions" is sub-attoamps.

Having an amplifier capable of picoamps is like measuring millimeters with a blank yard-stick. And nanoamps is like measuring microns with same stick. Can't be done.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[VEBill]

I once bought a one-gallon can full-to-the-brim of pico-amp diodes (leakage current much less than 1pA under some specified conditions). I was quite impressed with the leakage spec until I calculated the number of leaking charge carriers per second...

 

[MacGyverS2000]

This is one of those problems that simply can't be solved with COTS components. It would require some very custom work. Hell, I can imagine the leakage currents being many orders of magnitude larger than what's being measured. Talk about a needle in a haystack... and this is trying to find it when the lights are off.


Dan - Owner

http://www.Hi-TecDesigns.com

 

[Skogsgurra]

Absolutely!

And turning on the light will increase leakage.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[IRstuff]

The OP needs to:
1: throw the ions into something like a microchannel plate
2: use either conventional CCD or charge multiplication CCD for readout
3: TE cool the lot.

TTFN

FAQ731-376

 

[IRstuff]

Certainly, there are COTS photon-counting capable detector arrays, so it's not necessarily impossible.

TTFN

FAQ731-376

 

[Skogsgurra]

Photons are not ions. Photons have energy inversely proportional to their wavelength. Ions, i.e. elementary charges at rest have very little energy.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...