I would like to turn an Arduino clone into a current meter that allows me to measure the the power draw of a sensor node which has sleep and active modes. The sensor node's supply voltage is in the 4-6V range and during sleep the current I'm trying to measure is in the single-digit microamp range, while during transmission it may draw up to 150mA. Ideally I'd like to stay well below 1% error across the measurement range after calibration.
This type of problem seems to come up occasionally. Here and here are proposals for actively switching circuitry, and I've seen references to a device called uCurrent which also requires manual switching of sensing ranges. Then I came across this question where an answer suggested to use different-value shunts in series to accommodate the different sensing ranges, which struck me as an elegant solution, though I don't know enough to judge the solution compared to what was suggested in this question, namely to use multiple current sense amplifiers to measure voltage drop on a single shunt.
My question is: What are the relative merits of the two designs (multiple shunts + diodes vs single shunt with multiple amplifiers)?
For my use case I found a number of current sense amps that seem to fit the bill:
- common-mode input range beyond supply voltage
- typical offset voltages in the single-digit μV range
- typical gain errors below 0.1%
- gains of up to 1000
If I were to use a single shunt, I might use 50/500x gain current sense amplifiers in the INA191 family or 50/1000x in the INA21x family; connecting both a 50x gain and 1000x gain current sense amplifier to a 0.2Ω shunt, I could use two channels of the 14 bit ADC built into my my Arduino clone, measuring over the range 0-2V to sample up to 200mA with 24μA accuracy and up to 10mA with 1.22μA accuracy (assuming 13 usable bits from my 14-bit ADC).
If I were to use two shunts, I might choose two identical current sense amplifiers, perhaps with 100x gain, and appropriate shunts such as 0.1Ω and 10Ω. I'm also wondering if I might want to add a Zener diode to protect the ADC from overvoltages from the higher-resistance shunt.
Would one of the two designs yield better accuracy, or allow me to expand my measurement range noticeably, or be simpler or more reliable to build?
(edit: And would both be usable for high-side current measurement, which I understand is considered good practice and would allow me to re-use my current meter for more projects?)
I apologize in advance if this is not a good comparison for some reason; I'm still trying to understand some of the fundamentals. In that case I would appreciate pointers to relevant material.