R
Robert Baer
Guest
Louis Scheffer wrote:
MeThinks you meant 10^-16 amps!
How many *billion* AMPS???"Greysky" <greyskynospam@sbcglobal.net> writes:
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
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:
www.keithley.com/servlet/Data?id=6584
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
MeThinks you meant 10^-16 amps!