Measure Current at a Voltage above the Op Amp Supply Rail Using a Dedicated Current Shunt Monitor IC
Need to measure current through a sense resistor at 12V or 24V with an op amp working at 3.3V? Dedicated current shunt monitor ICs can do it!
In many cases, we are interested in monitoring the current draw of a system operating at 12V or 24V.
If the shunt resistor (R21) is placed between the power supply and the system drawing power, the voltage differential that needs to be measured to determine current flow will have a common mode component = to the supply rail voltage (12V, 24V, etc.).
The microcontroller and op amp circuitry that needs to perform the measurement is likely powered by 3.3V or 5V.
The vast majority of op amps are unable to measure voltages above their supply voltage — what to do?
Dedicated Current Shunt Monitoring ICs
Some current shunt monitoring ICs (for example: U9, NCV210) are able to measure common mode voltages above their supply rail.
For example, NCV210 can measure voltages = -Rail + 26V. So if you were to supply 5V and GND to power the IC, you could measure differential voltages up to 26V in magnitude.
Implementing the IC for current monitoring is super simple. Typically the IC has a fixed gain (as U9 does) or the gain can be selected with a set resistor.
Once the differential voltage is amplified by the IC, you can measure it with your ADC in the your microcontroller or USB DAQ to determine how much current is flowing through the shunt resistor.
Not For Use With Ultra Low Currents
ICs that can measure common mode voltages above their supply rail voltage typically have large input bias currents.
As such, these integrated current measurement chips are great for measuring mAs and Amps; but if you want to measure uAs, you’re going to need to use an instrumentation amplifier and measure voltages within your supply rails. You can see more on uA measurements here.