Resolve sub-millivolts with a PIC?
This is a one-off project.
I want to measure the output voltage of a peltier device. I have access to both wires. Voltage is under 10mV. I'd like 7 bits or so of precision. I said "precision". I don't care nearly as much about the accuracy, I can calibrate that out. Needs to have short term stability at fixed temperature long enough to get from calibration cycle to measurement cycle. I want to cobble this onto an existing project that uses a PIC16F627 to send data out the serial port. It has no A/D converter. I really don't want to add instrumentation amps to boost the signal anyway.
I've been looking into ways to generate a voltage with the PWM, stack the peltier device onto that, stuff it into the comparator...and "fix it in software". I fear that I'll never get the system noise down to the point where this is practical, but never hurts to ask.
Yes, I know that a couple of thermistors would be easier to manage electrically, but much more difficult mechanically. The Peltier device is just the right mechanical configruation for what I want. And I have it in my hand, as do I the pic16F627 system.
I'm not much interested in advice on using different devices or technologies. I can think of lots of 'em. I can get what I want with an IR thermometer and a calculator, but I'm bored with that. I'm interested in using what I already have to get me where I want to go.
A technique to measure small voltages would have general applicability to other situations.
So, any clever ways to resolve sub-millivolt signals with a PIC (without internal A/D) and not much else?
Thanks, mike