I'm wondering if anyone knows the accuracy of measuring uA current with a cheep (~25$) DMM? I need to measure some low power circuits (10-500uA) and I'm deciding whether I should get an analog uA meter or use my DMM.
My guess is that the DMM will not be very reliable? I can test it with a dummy resistor load and attempt to verify with a scope I suppose.
Update:
Accuracy I'm looking for: good question, I would ask the same. within 2.5% is good enough, what I want to try to avoid is disturbing the circuit…
Also The op-amp + shunt idea is one I had, I wanted to get peoples opinions first before I invested the time into that.
Best Answer
It all depends. What accuracy are you looking for? Some DMM have uA range, but the burden voltage is high. You can solve this easily using an external circuit. All you need is a resistor and an precision OpAmp.
Dave Jones from the EEVBlog designed this circuit that lets you use your DMM in DC Volts mode to measure precisely nA, uA and mA with good resolution. You can find the schematic and pcbs designs files in: http://www.eevblog.com/projects/ucurrent/
If you don't want to build the whole circuit it's fine, you can just use the uA part to get a good reading. Just find a suitable resistor and an OpAmp with similar specs. It depends on the bandwidth (how fast the current will be changing).
I have built several of these myself and they work great.
Another solution is to buy some of the Silabs gecko starter kits that include a current measuring circuit (they cost $29USD). It's easy to connect your circuit to those stater kits and via their software you can do several power calculations. You can even see a real time graph.