A CMOS INTERFACE FOR THERMOCOUPLES WITH REFERENCE-JUNCTION COMPENSATION

This paper presents a CMOS interface for a thermocouple, including ref erence-junction compensation. The interface contains a first-order osc illator whose period is modulated by the signal from the thermocouple (V-x) and two other signals generated internally. One is a base-emitte r voltage (V-BE) and the other is a PTAT voltage (V-PTAT). Linear comb inations of the periods corresponding to these two voltages are made b y a mu C, to obtain a reference signal and a temperature-dependent sig nal. The internal signals are required to provide auto-calibration for offset and gain and to measure the interface internal temperature. Dy namic Element Matching is applied to generate an accurate and reliable PTAT voltage. The inaccuracy in the base-emitter voltage is calibrate d using a vertical PNP substrate transistor with multi-emitter areas. The applied modulator used in the circuit has a second-order filtering which suppresses low-frequency (l/f) noise. This filtering property e nables the use of a low-cost CMOS process for the implementation of th e circuit. The interface is able to measure a voltage (V-x) in the ran ge of -27 mV to 100 mV, the measured accuracy of the system over a tem perature range of -25 degrees C to 75 degrees C is 550 ppm of V-x +/- 2 mu V. The inaccuracy in determining the reference-junction temperatu re is 0.6 K. The measuring time is less than or equal to 50 ms.