Detecting currents less than one femtoamp?

Anyone know of a reliable way of detecting currents less than one femtoamp i.e. 1X10^-16 amp full-scale? I figure to perhaps use some instrumentation amps air wired (no pc board) but have not had much luck getting reliable readings below about a nano amp - and at this scale, even an accidental breath can leave a film on the chip that creates a current path that swamps the signal I am looking for.... some have suggested using lots of acetone for cleaning the circuits, and using silver (or even gold) solder... others have suggested measuring for resistance and figuring it from there.... I get the feeling low current designs seem to be more an art than a science. Any hints, pointers, or guffaws appreciated. Thanks

Greysky

Reply to
Greysky
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"Greysky" wrote

Look under 'electrometers'.

1 fA = 1e-15, ?. Are you looking for 0.1 fA FS?

1A = 6.2e18 electrons/second.

1fA = 6,200 electrons/second.

You are not looking at current, you are counting electrons/second. It can be done, however.

--
Nicholas O. Lindan, Cleveland, Ohio
Consulting Engineer:  Electronics; Informatics; Photonics.
Reply to
Nicholas O. Lindan

What's the signal source? Can you use a lock-in trick?

John

Reply to
John Larkin

There are old instruments called 'electrometers' that are routinely used in this current range. The application notes associated with them discuss lots of common problems and how to work around them. See for example:

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The usual techniques involve lots of cleaning, guarding (surrounding the measurement point with wires driven to the same potential), special cables such as triax, avoiding certain materials (watch out for the photoelectric effect!), control of humidity (if you can), and so on.

One electron is 1.6x10^-19 coulombs, so 10^16 amps full scale is about

600 electrons/sec full scale. So your measurements will definitely be noisy and extreme care at controlling leakage will be required.

Lou Scheffer

Reply to
Louis Scheffer

I can understand about the complexity of trying to measure such a low current. There are many outside influences that can greatly effect the measurement accuracy.

Try posting to, sci.physics. I have found some type of people over there that are excellent with involved problems of this catagory.

--
Jerry G.
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Reply to
Jerry G.

femtoamp

instrumentation

reliable

science. Any

Here's an application note from Keithley talking about low current measurements in general and the precautions needed to make them. In particular, the chart that shows the expected size of various type of unwanted effects is eye-opening.

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Lou Scheffer

Reply to
lou

How many *billion* AMPS??? MeThinks you meant 10^-16 amps!

Reply to
Robert Baer

And ionizing radiation sources (e.g., don't be wearing an old glow in the dark watch)

You've already noted problems with convective currents carried by air flows as well as the related problems of surface contaimination, doing the whole thing in a DRY nitrogen-filled glove box may be useful to avoid these issues.

SOmeone else pointed out that phase-lock or lock-in techniques can be useful if you can use them. If your source is DC, you can still use lock-in techniques -- using a chopper. Switched capacitance or pumped capacitance frontends can give both gain and chop DC or low frequency sources in preparation fo rlock-in detection.

I'd suggest you beg/borrow/buy a Keithley to start. All the electrometers of theirs I've seen allow you to use the electrometer as a front-end and feed an amplified/buffered signal to whatever you want to use. Even if you need more than what a Keithley electrometer alone can provide, the instrument can still be useful as part of the setup. ANd their documentation is a crash-course in low level measurement.

On-line, find Bob Pease's columns -- he had a good (though short) one on low current measurements a few years back, it was still available in the archives last I looked.

Visit the supermarket and buy several rolls of aluminum foil. Get regular and ultrawide and also get the heavy duty. For the price it is just worth having on hand. ANd it is so useful for impromptu shielding and or guarding electrodes. N.B., you cannot expect to make a good solder connection to aluminum foil -- have lots of short jumpers will alligator clips on both ends.

Reply to
Kevin G. Rhoads

I read in sci.electronics.design that Kevin G. Rhoads wrote (in ) about 'Detecting currents less than one femtoamp?', on Wed, 6 Oct 2004:

It's no great problem in dry conditions. Solder a wire to a scrap of copper foil and use a paper staple to fix it to the aluminium foil.

If it gets wet, the juxtaposition of the four metals (copper, tin or zinc plating on the staple, iron in the staple, aluminium) sets up rapid electrolytic corrosion.

--
Regards, John Woodgate, OOO - Own Opinions Only. 
The good news is that nothing is compulsory.
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Reply to
John Woodgate

femtoamp

instrumentation

Let's see..

1 mA = 10^-3 1 uA = 10^-6 1 nA = 10^-9 1 pA = 10^-12 1 fA = 10^-15 _not_ 10^-16! [snip]

Reply to
Watson A.Name - "Watt Sun, th

femtoamp

Some engineer! No matter what the number of electrons, it's still a current.

Reply to
Watson A.Name - "Watt Sun, th

femtoamp

instrumentation

reliable

accidental

swamps

acetone

others

there.... I get

science. Any

used

the

cables

photoelectric

Lots of teflon!

I thought electrometers involved fairly high voltages.

noisy

Reply to
Watson A.Name - "Watt Sun, th

Err....Teflon is *NOT* good; see what Bob Pease has to say... BTW, if the currents get *really* low,then use of Millikan's oil drop experiment, or some reasonable equivalent - and *count* them!

Reply to
Robert Baer

On Sunday 10 October 2004 01:48 am, Watson A.Name - "Watt Sun, the Dark Remover" did deign to grace us with the following:

Yabbut, "current" kinda implies a smooth flow, like a river. What they're talking about here would be much ... lumpier.

Thanks, Rich

Reply to
Rich Grise

He said LESS than 1fA. 10^-16 is 100 attoamperes

It is doable with discrete front end of I/V op amp configuration. I used

3N155
--

    Boris Mohar
Reply to
Boris Mohar

Tell us about your observed current offset and drifts. (The 3n155 p-channel MOSFET is spec'd at 10pA maximum gate leakage, for Vgs = -25V forward, admittedly a rather silly and basically useless spec.)

Also, did you use any input protection circuitry? What was the size of your feedback/sense resistor? Where did you get it?

--
 Thanks,
    - Win
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Reply to
Winfield Hill
[snip]

A few weeks ago I tried an aluminum soldering technique that someone mentioned here. I used a big 100W iron, and sanded the spot and then put a drop of Oatley flux on it to keep the air away. The original advice was to use oil, but the flux seemed to work okay. Anyway, the aluminum got hot several inches away, but the solder flowed right into the alumninum, wetting it nicely. With foil, it shouldn't take so much heat, so a smaller iron should work okay. A little experimentation might be worth a lot.

I was reading an article (in Popular Science?) about how mercury can corrode a huge aamount of aluminum, and it showed an I-beam that had been exposed to it for less than a day. The I-Beam was eaten away, and turned to powder. It said something about how it could destroy an airplane. Weird!

Reply to
Watson A.Name - "Watt Sun, th

Dark

You make it sound like the lumps are big enough to cause variations in the measurement. I would like to think of the 'lumps' as the particles in talcum powder, so small as to be undetectable. But the particles in talcum are huge compared to electrons.

But one point I forgot to mention is that these measurements are analog, not digital. Therefore, measuring, rather than counting electrons is more appropriate.

Reply to
Watson A.Name - "Watt Sun, th

Dark

used

What's eye-opening is that at these ultra low currents, just a few picofarads hold so much charge that it greatly affects the whole measurement.

Reply to
Watson A.Name - "Watt Sun, th

Useless spec indeed. Actually we used 3N155A but do not have the data sheet at the moment. We hand selected them for low Vgs because Igs varies exponentially with Vgs

No. This was for ionization chamber which had the signal electrode enclosed by the polarization electrode (can)

1T Ohm in T feedback configuration. It was an I/V design.

The best at the time were from

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Some others showed an inverse voltage coefficient. Manufacturer tested then at 10 volts and higher but when we tested them at lower voltages the resistance would start to increase rapidly. Of course it is not possible to determine what the resistance is at 0V but when the electrometer is zeroed this is what the feedback resistor sees.

The other big headache was the humidity. This was solved by potting the front end in soft wax. We tried some fancy transparent potting gels but they leaked. Potting also prevented parasitic ion chamber effects caused by the interaction of nearby power leads and the input.

Regards,

Boris Mohar

Got Knock? - see: Viatrack Printed Circuit Designs

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void _-void-_ in the obvious place

Reply to
Boris Mohar

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