> the bridge.  This voltage is read by the AD7714 (A/D converter, the
> output of which is converted to a serial word) and if it is not zero
> (a balanced bridge), then I can create a correction signal according
> to the size of the error (proportional plus integral controller), and
> apply or reduce the heat until the thermistor bridge is again balanced.
> During all this, I don't really need to know the number of degrees that
> the thermistor is off from the setpoint, I just need to know the bridge
> voltage and take the necessary action to bring it back into balance.
>

This is fine if the thermistor / measuring circuit is reasonably linear
over the range you intend to use, but if it isn't, the gain of your
control loop will vary with the setpoint. You may need to take this into
account when optimising the PID parameters.

-adrian