I want to design an autoranging voltmeter with a PIC microcontroller and its ADC.
- Measure voltages from +0.2v to +50v
- Protect against reverse inputs or slightly higher voltages
- Input impedance can be quite low, drawing up to a few mA.
- Accuracy of around 1% or better (although not necessary for smaller voltages)
- Sample time of around 0.5ms or better
I have a circuit in mind, and would appreciate feedback. Perhaps there are simpler circuits I could use. The PIC will be on a 5v supply. I would probably use a basic, cheap op-amp running from 0v and a +12v rail. The tristate uC outputs would either set the input voltage dividers or alter the gain of the op-amp circuit to higher than unity.
Does this look reasonable?