Paul said:
I'm trying to think of methods that may detect ~ 1/2 picoamp DC
without applying any appreciable *bias* current.
I have a passive analog amp meter that with the aid of a microscope
can accurately detect down to 100 pA. Close, but no cigar.
Any ideas is greatly appreciated.
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.