Measuring sub picoamp DC by electrical or chemical means

Do you have a link (or a paper reference)? I'm interested.

DB

follower?

You are talking about the preamp, correct? The preamp cannot have appreciable voltage offset or bias current relative to the voltage or current levels one is trying to measure. So what I was saying is that my 0.5 uV fed into the INA116PA op-amp would cause 5E-22 amps of current due to the op-amps extremely high resistance. So the highly sensitive INA116PA op-amp could not detect 0.5 uV. I think Keithley did an amazing job, as the Keithely electrometer you pointed out can measure down to 200uV.

A MOSFET would do trick with their ~ 10T ohms and higher resistance from base to source. I believe an IRF540 has close to 9T ohms from base to source.

David A. Smith

-

Thanks David and John. That's actually a great idea, and applicable to this design. I've never tried that method. Google patents has an interesting patent describing such a method-- Figure 2. They use a FET to reset the cap, but I'd imagine John's idea of using a glass encased reed relay would help reduce noise.

Regards, Paul Lowrance

The general concept of an integrate and hold/reset is shown on page 4 of this application note:

Note that with a current input instead of a voltage input you don't need R1. And with such a low bias current opamp as the LMC6001, you also don't need R2, so it is a pretty simple circuit, except for the logic controls for the the input and capacitor dump -- neglecting the supply and bypass, just an opamp, a capacitor and one or two reed switches.

Be sure to get one that has a magnetic shield over the coil, so you don't induce voltage spikes in the input circuit when the coil switches.

Relays with the ends of the glass capsule sticking out of the coil forms used to be common, but I don't see many, any more. You may have to buy the switches and wind your own coils around them. Got any 40 AWG wire?

The first idea is to go talk to a real electronic engineer and explain your system to him!

Look. There is much MORE to "1/2 picoamp DC" than just current! One needs to know the voltage and the source impedance of your equipment. For example there are many Keithly meters that can measure extremely low currents. However they ONLY work with relatively high voltage sources. They use very large (high value) precision resistors to get a reasonably measurable voltage from a tiny current. But in the measuring range they have to develop a decent fraction of a microvolt as an output signal. Some sources don't have this voltage available and the whole thing fails.

The best way is probably to use a MOSFET (the kind with no protection diodes!) and then simply use your source to charge the input capacitance of the device as someone already suggested. The slope of the charging voltage gives the current. If it's too sensitive you can add input capacitance in parallel which increases the known accuracy. Just just a manual discharge.

Get help.

Thanks, David and John. I'm familiar with the concept, but never thought of it in terms of measuring charge.

DB

Thanks for the tips. I'm having success simulating the LMC6001A op- amp.

Paul Lowrance

I wonder if, since you are talking about only 10^7 electrons, that you could not apply this to a heated vacuum tube plate and *count* the durned things?

David A. Smith

First refer to the "Johnson Noise Limit". I worked with DC circuits (for measuring very small amounts of thermal changes) about a factor of 10 above that level. It took a nastily long time for the system to get to a relatively quiet equilibrium before the small emf produced in the DC circuit could be detected above the noise level. FK

I think from what you have described you want the sort of current to voltage converter that is typically used in mass spectrometry in conjunction with Faraday cups to measure ion beam currents. ISTR the ones I worked with were usually setup to give 1v out for 10pA current in (but it was a long while ago) and then measured on Solartron 7060 voltmeters at 6x9s precision 1s integration under computer control. The amplifiers had to be thermally managed and heavily insulated (and they lived in a vacuum) - thermal gain calibration and baseline drift was always an issue.

Breif description of MS hardware at:

They were good to sub-picoamp at around 10^-13A but needed careful handling. The 10^11ohm resistors were exotic temperamental from batch to batch and there had to be fancy feed forward compensation to allow the amplifier to cope with transient signals reliably. I expect a search of Faraday amplifier will get you some patents. Some of the tricks were trade secrets - though you could try asking on sci.electronic.design.

Regards, Martin Brown

Do you know of a distributor for these reed switches? Mike

They list 409 hits for the search parameter [reed switch], including some of the ones shown in that data sheet

--
"I'm never wrong, once i thought i was, but was mistaken"
Real Programmers Do things like this.
http://webpages.charter.net/jamie_5

I'll take a look, I liked the .2pf capacitance, I'm assuming that's across the contacts open circuit? Mike

(snip)

I don't know what else it could be.