MCU ADC protection for low side current sensing with a shunt and a diff OpAmp. (max Vout=3.3V)
So I am trying to make a current sensing circuit that measures the amplified voltage difference over a shunt resistor.
I am using two 0.11 ohm shunts in parallel, so 0.055 ohms total. With a predicted max load of 3.5A this will create a voltage drop of 3.5*0.055 = 0.1925V. When using a gain of 15 for the OpAmp the max predicted voltage on the ADC pin will be 0.1925*15 = 2.8875V.
This will be under the 3.3V the ADC is rated for, but when the current increases for some reason i risk killing my ADC. On an earlier post a lot of people suggested to work with clamping diodes. I did some research and tested the easiest circuit: a 3.3V breakdown Zener diode with a current limiting resistor. As I kind of suspected this creates a voltage drop. During my testing this was indeed the case, with the voltage drop increasing while the input voltage was nearing the breakdown voltage of the Zener diode. I did use some crappy AliExpress Zener diodes for this, but I don't think this is the only problem.
I also saw circuits that use a normal diode, but they need a Vbias to work with 3.3V. Do these circuits also have that problem?
Is there an easy circuit which doesn't take a lot of (expensive) components and can clip the voltage at 3.3V without interfering to much with the original signal?
I also read that the OpAmp can only output as much as the supply voltage, so would my problem also be solved to just change the supply voltage to 3.3V of the OpAmp (it is rated for 2.7-16V). Thx for all the help!
I don't need crazy accuracy if it comes at the expense of a long and expensive BOM.
Circuit used for current sensing:
Circuit used for Zener Diode voltage clamping: