Detecting currents less than one femtoamp?

[Greysky]

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

 

Greysky wrote:

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 - [...] I get
the feeling low current designs seem to be more an art than a
science. Any
hints, pointers, or guffaws appreciated. Thanks

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.

http://www.keithley.com/main.jsp?action=topproductlinks&mn=6514&link=Documents#subtyp1
Lou Scheffer

 

[Kevin G. Rhoads]

(watch out for the photoelectric
effect!),
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.

 

[John Woodgate]

I read in sci.electronics.design that Kevin G. Rhoads
<kgrhoads@alum.mit.edu> wrote (in <4163EE81.D5AF7646@alum.mit.edu&gt
about 'Detecting currents less than one femtoamp?', on Wed, 6 Oct 2004:

you cannot expect to make a good
solder connection to aluminum foil -- have lots of short jumpers will
alligator clips on both ends.

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.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

 

[Watson A.Name - \"Watt Su]

"Rich Grise" <null@example.net> wrote in message
news:QO9ad.2874$ua2.9@trnddc09...

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


"Nicholas O. Lindan" <see@sig.com> wrote in message
news:TGy8d.4395$Vm1.3257@newsread3.news.atl.earthlink.net...
"Greysky" <greyskynospam@sbcglobal.net> wrote

Anyone know of a reliable way of detecting currents less than one
femtoamp
i.e. 1X10^-16 amp full-scale?

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.

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

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

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.

Thanks,
Rich

 

[Louis Scheffer]

"Watson A.Name - \"Watt Sun, the Dark Remover\"" <NOSPAM@dslextreme.com> writes:

"Rich Grise" <null@example.net> wrote in message
news:QO9ad.2874$ua2.9@trnddc09...

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

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.

They certainly won't be undetectable. At 10^-16 amps = 600 electrons/sec
even a 1 seconds measurement will see +-25 electrons RMS, with
larger peaks. That's about +- 4% on each measurement. If you try to measure
in 1/10 second it's much worse = 62 electrons +- 8, for a +- 12% variation
between any two measurements.

Lou Scheffer

 

[Robert Baer]

"E. Rosten" wrote:

Watson A.Name - \"Watt Sun, the Dark Remover\" wrote:

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!

That is correct. Metals are often very good solvents for each other
(which is why alloys work well). Aluminium dissolves in mercury. When
the aluminium becomes exposed to oxygen in the air, it oxidises as
usual, but it is unable to form a protective hard coat since it isn't
sitting on a nice, solid base. Since the mercury essentially operates as
a catalyst, it can do pretty much unlimited damage.

-Ed

--
(You can't go wrong with psycho-rats.) (er258)(@)(eng.cam)(.ac.uk)

/d{def}def/f{/Times findfont s scalefont setfont}d/s{10}d/r{roll}d f 5/m
{moveto}d -1 r 230 350 m 0 1 179{1 index show 88 rotate 4 mul 0 rmoveto}
for /s 15 d f pop 240 420 m 0 1 3 { 4 2 1 r sub -1 r show } for showpage

In regard to soldering aluminum and solubility:
When i first wanted to solder to aluminum, i bought some solder made
for that purpose.
It discovered that it had cadmium, so i stopped using it.
Then i experimented with 60/40 solder, hotter than normal irons, and
various fluxes.
This was over 30 years ago, and i think the best flux was ammonia
chloride.
Net result: it is easy to solder to aluminum, provided that one can
easily break thru the oxide coating while preventing more to be created.
However, i also found that aluminum is rather soluble in solder, and a
depression is made where the solder is applied.
I also found that once the barrier is broken, the solder can flow
underneath the oxide skin and spread easily over a wide area (looks
wierd, seeing a moving liquid "lump" under a "scum").

 

[Robert Baer]

Rich Grise wrote:

On Monday 11 October 2004 06:50 am, E. Rosten did deign to grace us with the
following:

Watson A.Name - \"Watt Sun, the Dark Remover\" wrote:

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!

That is correct. Metals are often very good solvents for each other
(which is why alloys work well). Aluminium dissolves in mercury. When
the aluminium becomes exposed to oxygen in the air, it oxidises as
usual, but it is unable to form a protective hard coat since it isn't
sitting on a nice, solid base. Since the mercury essentially operates as
a catalyst, it can do pretty much unlimited damage.

I've also heard that mercury is one of very very few things that can
dissolve gold. Presumably, that's why it's used in tooth fillings.

Cheers!
Rich

...and silver fillings.