Circuit Project Electronic

Accuracy depends only on the tolerance of the nominal 10-to-1 R1/R2 ratio. Temperature-to-digital conversion involves a voltage-to-frequency-ratio scheme: Difference amplifier IC3 compares the square wave from Q1’s emitter to the signal developed across R3 by switches IC2A and IC2B, acting concurrently with R4, R5, and the 2.500V reference IC4 develops. IC3 amplifies the difference with a gain of 250 and applies the result to synchronous rectifier IC2C.

Read more original source:
http://www.edn.com/archives/1996/050996/10di3.htm