The LM4120 is configured to produce current 0f 1 mA through the silicon diode. The reference part is essentially a voltage regulator producing a precise 3.3 volts across the 3.3K ohm resistor. The current through this resistor is thus 1 mA. The total current through the silicon diode is the sum of the 1mA plus the GND lead bias current of the LM4120.

At a constant current the forward voltage across the diode will be nicely linear over a working temperature range from -55C to +125C. It should be possible to achieve linearity to around a degree or so (if my memory from a project I did years ago serves me correctly). The circuit output voltage is about 0.6 volts at room temp and will go down for higher temps and go higher for colder temps. The voltage change is about 2.2 mV / deg C. A 12 bit A/D works reasonably well but it may be advisable to condition the signal via some gain at the input of the A/D converter. If the conditioner has a negative gain the response slope measured by the A/D converter will match the temperature change slope.

Michael Karas