[Groth]

Originally Posted by Incoherent

The heater current will max out at about 8-10amps, probably a little high to be on board but I am a little worried about connection resistance buggering up the readings. What do you think? The currents around this circuit are very low (10-100µA area), probably a prime candidate for a bit of inductive crosstalk. Any thoughts on reducing this if I did keep it on board. It is all effectively DC of course.

With the currents from the sense shunt to the amp so low, connection resistance isn't a big issue there (mV signals, µV connection drops). As long as you use twisted pair from the shunt to the board (to avoid differential noise), just let the in-amps reject any common mode noise that shows up.

You are right regarding digital i/o, it'd be a bit silly to not give myself at least the future possibility.
I am debating now whether to optocouple the digital side. The added circuit complexity does not appeal.

Give your self the expansion possibility, and put aside digital thoughts for another day.

Been looking at precision resistors, they are not cheap (~$3 each here in Sweden) for 0.1% <5PPM/*C metal film. I'm wondering whether your frypan sorting technique will be applicable here if I used the far cheaper 1% 50PPM/*C ones.

I think you'll do fine without the precision ones. The thermistors will all want to be calibrated anyway, so the 80K's don't need to be precision. The juctions of R13/R15 and R11/R12 can simply be measured - no need for precision. R9 would be cool to have precise, but there's no point in having it more precise than R10...

Go cheap.

Been thinking, actually theres no real reason why I can't use a 5v reference, If I am reading the MAX186 spec sheet properly, that way I can power the INA122s from Vcc.

I didn't read the datasheet closely, so :shrug: Check to see how the ADC inputs respond to voltages higher than Vref. Most have a limit to how much over-voltage current they'll sink, be sure the caps on the inputs won't supply more that that.