Can someone recommend the simplest circuit for temperature

measurement with a passive thermistor. I also need the theory

and the mathematics. I recall that I need two clibration points

and two variable resistors.

The simplest would probably just be a voltage divider with the

thermistor as the lower (ground side) resistor. A starting point would

be to use an upper (Vcc side) resistor that's around 1/2 the nominal

value of the thermistor, giving you a voltage range from about 2/3 of

Vcc down towards ground as the thermistor heats up.

Measure the voltage, calculate the thermistor resistance as

Rt = R1 * Vm / (Vcc - Vm). Look up or calculate the temperature as a

function of the resistance. That can range from a simple linear

approximation, to a two- or three-segment linear approximation, all the

way up to a Steinhart-Hart approximation (google for it).