A smart CMOS interface system for thermocouples

This paper presents a novel interface for voltage-generating sensors, such as thermopiles and thermocouples. The processor converts sequentially a DC input. Voltage and three internally generated voltages into a period-modulated square-wave output 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 applied modulator used in the circuit has a 2nd-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. The inaccuracy in determining the reference-junction temperature amounts to 0.6 K. The measuring time is ⩽50 ms