Hi to all,
often i have to measure the current absorbed by small circuits,in battery appplications Normally i deal with consumptions big as 20mA in run mode and 2uA in sleep mode.
One method that sometimes i use is to desconnect the battery,leaving the circuit feeded only by a 10,000 uF capacitor,seldom and quickly measured by an high impedence multimeter,(of course not a 10 Mohm oscilloscope probe).Measuring the difference in voltage at a specified times i can calculate the consumption.The cap was previously measured,to avoid tolerance error.
More often i need a more immediate measurement,so i feed the circuit by a commutator switch,in parallel to 1,10 and 100 ohm shunt resistors. The latter is constantly inserted so that the circuit is not completely relying on itself during commutations. When the uc goes to sleep and i need to have an hopefully realistic rading of few uA i switch on the 100 ohm shunt,the voltage drop between battery and circuit is below the mV,therefore irrelevant. The voltage is read by a 6 and half digit DMM via a 60cm(2 ft)twisted wire.
Is this a good method or others can be better?
Is there any advantage using an instrumentation op amp as buffer or amplifier, wired very closely to the shunt?
Many thanks for your attention
Diego