Apropos of the 'Digital Multimeters' thread, I've just been running into the problem of getting enough current resolution on my DMMs. The dedicated current ranges tend to have resolutions in the tens of nanoamps or worse, which isn't adequate.
Of course the classical trick is to use the lowest DCV range for current sensing. Most DMMs have a 10 Mohm input resistance on their voltage ranges, so a 4.5-digit meter on its hundreds-of-millivolts range has a current resolution of
10 uV/10 Mohm = 1 pA, far better than on their dedicated current ranges.That works perfectly well, but it's a mild pain with photodiodes, which are sensitive to burden voltage. You can put in a parallel resistance to keep the burden voltage low enough that the photodiode's forward conduction doesn't cause measurement errors. (In practice, 100-200 mV won't cause significant errors with a silicon photodiode in most cases.)
My Keithley 177A has nice low-current ranges, and I also have their models 610C, 602, and 410 analogue picoammeters; I can also easily slap together all sorts of custom current amps.
In testing products for patent lawsuits, however, it's super helpful to have instruments from well-known manufacturers that just read what you're trying to measure directly. Today I was measuring nanoamp level currents from green-filtered photodiodes, and it would have been more convenient to have an actual current range with resolution down to 100 pA or 1 nA.
Suggestions?
Thanks
Phil Hobbs