I'm building a piece of automatic test equipment to be used in-house and have a need to measure relatively small currents (e.g. 3 µA). So I'm planning on measuring the voltage across a 1% 1K resistor (3 mV) and multiplying it by 100 or so (in two 10x stages) and then feeding it into an 12-bit ADC with a 4.096 V reference (so one bit = 1 mV).
I picked what I thought was a suitable op amp for this task (ALD1722), and hooked it up in a non-inverting amplifier configuration (Rf = 9.09K, Rg = 1K) with a +5V supply, and discovered for input voltages in the tens of millivolts I got a value close to what I expected but as I decreased the input down to a few mV or so the output went to 0.
Puzzled, I hooked up the same op amp in a unity gain configuration, and discovered that the output was lagging the input by about 10 mV; i.e. an input of 50 mv results in an output of 40 mv, and any input below 10 mV results in no output.
I'm a mostly digital guy, so op amps have always been a little mysterious. (I never studied IC op amps in class, as the 741 wasn't even released until my senior year in college.) I picked this op amp because the datasheet says it has a typical Input Offset Voltage of 25 µV, so I wasn't expecting this kind of error, but maybe I don't understand what offset voltage is.
Is there some sort of biasing circuit I can add to correct for this, so I can feed mV or smaller voltages into the op amp and get a 10x output?