On Tue, 28 Jan 2003, Sean H. Breheny wrote:

*>What about putting a large resistor in series and measuring the voltage
*>drop across it?

Problem: large resistor ~= meter input impedance on volts scale -> bad bad
errors.

The way to go is to use a Gohm or higher (you can go up to 10^13 ohms with
off the shelf opamps now) input opamp and feed the current directly
between its + and - inputs, with a suitable (can be Gohm range) feedback
resistor. This is 5 orders of magnitude more sensitive than a usual
voltmeter (which has 10Meg input impedance). By swtiching the inputs
between them (with reed relays or other non-leaky devices) the offset etc
can also be compensated or accounted for. Measuring such low currents
requires very careful work. Dirt, (antistatic) coatings and flux residue
on cables and front panels can conduct more then the circuit and falsify
readings, you have photoelectric effects with certain materials, plastics
behave like electrets, moving cables generate voltage on account of their
charge & your changing the capacitance wrt ground/you by moving them etc.

Peter

--
http://www.piclist.com hint: The list server can filter out subtopics
(like ads or off topics) for you. See http://www.piclist.com/#topics