Here’s a
design circuit that is used to active circuitry and
an analog meter to make sensitive DC current measurements. A reader
subsequently asked if it could measure AC microamperes and that is what spawned
the idea. This is the figure of the circuit;
While the schematic is similar, there are notable
differences—the voltage drop is now 100mV rather than 1V, the sensitivity is
10nA full scale rather than 100nA and it now measures AC or DC. A precision
rectifier rectifies the AC voltage so that a DC meter may be used. Actually,
this is wishful thinking because the sensitivity is too great for my set-up. As
a result, the meter reads half-scale (5nA AC noise) without anything connected.
The 100nA setting is much more reasonable—Mr. Marian obviously knew what he was
doing by limiting his circuit to 100nA FS. 10nA can still be measured, but the
meter would be reading 10% deflection on the 100nA range. All would work better
if packaged inside a shielded enclosure. On the 10nA range, it is so sensitive
that AC current caused by body capacitance at a distance of 30mm from the input
lead was easily visible—something like putting your hand near a high impedance
oscilloscope probe and watching the AC pickup on the trace.
