Measuring currents smaller than 1 uA
Hello, this is a bit off topic for this forum, but I couldn't come up with a better place to ask it...
What are some good ways to measure currents smaller than 1 uA? The purpose would be to make sure that a micro has gone into a very-low-power sleep mode, and to distinguish various sleep-mode settings. The test setup needs to pass higher currents (tens of mA) without appreciable voltage drop, to power the PIC before it goes to sleep.
My ordinary multimeter has a resolution of 0.1 uA on its lowest range, which probably means its accuracy is 0.2 uA. And when measuring the current to a sleeping PIC, it often reads 0.0.
I can think of various tactics such as a resistor with a switch across it -- when the PIC has gone to sleep, open the switch and measure the voltage across the resistor. Or maybe a resistor in parallel with a diode so that with appreciably higher current, the voltage drop still won't exceed 0.6 V or 0.2 V (Schottky); then measure the voltage across the resistor. But I haven't tried it.
Do I need to either spring for a high-end benchtop multimeter or build myself an instrument? Other ideas?