Hello,
There are a number of design issues associated with your analog measurement requirements.
You will first need to determine the analog interface circuitry required, which may extend beyond a simple voltage divider. If the analog signals originate from the "outside world" you will surely need to protect the MCU against high voltage transients. Additionally you will need to consider the source resistance of these signals - to achieve sufficient accuracy in conjunction with12-bit resolution, the signals will each need to provide a low source resistance to each ADC input.
More than likely additional voltage follower buffers would be necessary, to be interspersed between the voltage divider and the ADC input. The voltage divider would need to be of sufficiently high resistance for negligible loading of the source, dictating the use of operational amplifiers with very low input bias current. Without knowing the nature of the signal sources, I cannot be any more specific.
The initial accuracy of the voltage divider components can be compensated for if a calibration process occurs during production, so that correction factors can be determined and stored.
For the smaller pin count MCUs, the ADC reference will be internally tied to the Vdd and Vss rails. Any variation of the supply voltage, with temperature or load, will affect the accuracy of the measurements. It is possible to compensate for long term Vdd variations by also measuring a separate internal bandgap reference within the MCU. You will need to determine whether the Vdd supply rail is sufficiently stable.
Since all MCUs generate internal noise, this can cause problems, especially for 12-bit conversions. One possible approach is to put the MCU into lower power wait mode immediately after a conversion is started, and then use the ADC interrupt to wakeup and read the result. Another method might be to take multiple readings of each signal, and use the average result.
If you cannot achieve the desired accuracy using the internal ADC module, you might need to use an external ADC device that communicates to the MCU, typically via SPI. This can be used inconjunction with an accurate external voltage reference.
Regards,
Mac
