I have thought a bit about your application in the last days and would like to give some hints:

Your Silabs C8051F020 micro provides a reference voltage of 2.43V typical (2.36...2.48V). Additionally taking into account offset voltages of OPamps and tolerances of resistors, I would convert the input range of +/-10V to an ADC input voltage span of 0.1V...2.3V (refering to a reference voltage of 2.43V), providing some headroom at the limits. So, my divider looks a bit different from yours:



Chosen resistor values give a span of 0.12V...2.27V, actually. Tolerances of divider resistors should be less than +/-1%, of course!

The TLC274 is powered by +5V/0V and should be available where you live. It will also profit from an input/output voltage range that goes not too close to the lower supply rail (0V). By the way, there's the TLC279 available, which shows very low offset voltage!

The buffer buffering the reference voltage can also be used to provide a simple voltage clamp for the ADC. Input voltages must not exceed the supply voltage of micro (which is about 3.0...3.3V in your application) by more than 0.3V. Due to the clamp, the input voltage cannot be higher than ca. 2.4V + 0.6V = 3V.

The circuit also contains a ceradiode at input, a very smart ESD/surge protection device, showing very low leakage current and junction capacitance, just the right thing for an analog input. You can also see a 1nF cap providing some low pass filtering. (You can increase this value, if desired.)

The TLC274 isn't very fast, so its finite settling time might require a rather high aquisition time of ADC, which you can fortunately adjust for the C8051F020.

The TLC274 provides internal protection diodes at inpus and outputs, which can serve as overvoltage protection, if the according currents are limited (done by the 82k resistance).


How to calibrate the circuit?
By software, of course!

Example: Provide a jumper and a button to free port pins of micro. Code of micro shall read this jumper at each start. If set, it shall wait for the button being pressed and take the adc readings as reference for -10.0V input voltage, which you have applied before. The next button pressing shall represent +10.0V at the inputs, which you have applied before. Store the readings "permanently" anywhere you want and take them for all further ADC corrections.


Kai