Abstract
This paper presents a CMOS interface for a thermocouple, including reference-junction compensation. The interface contains a first-order oscillator whose period is modulated by the signal from the thermocouple (V_x) and two other signals generated internally. One is a base-emitter voltage (V_BE) and the other is a PTAT voltage (V_PTAT). Linear combinations of the periods corresponding to these two voltages are made by a µC, to obtain a reference signal and a temperature-dependent signal. The internal signals are required to provide auto-calibration for offset and gain and to measure the interface internal temperature. Dynamic Element Matching is applied to generate an accurate and reliable PTAT voltage. The inaccuracy in the base-emitter voltage is calibrated 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 (1/f) noise. This filtering property enables the use of a low-cost CMOS process for the implementation of the circuit. The interface is able to measure a voltage (V_x) in the range of −27 mV to 100 mV, the measured accuracy of the system over a temperature range of −25°C to 75°C is 550 ppm of V_x ± 2µV. The inaccuracy in determining the reference-junction temperature is 0.6 K. The measuring time is ≤50 ms.
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