A Precision Integrated Interface Circuit for Thermopile Based Sensors

This paper presents an interface circuit, which is optimized for reading the output signal of thermopile-based sensors. The target sensor has an output voltage less than 10 mV and an output resistance of about 40 kOmega. In such a system, errors come from various sources, e.g. limited settling time, components´ offset, 1/f noise, thermal noise, long term drifting as well as the noise from the sensors. To achieve the highest possible signal-to-noise ratio, different levels of error correction techniques have been applied. At the system level, three-signal autocalibration technique and oversampling with digital filtering have been applied. At transistor level, the geometry and biasing conditions have been optimized. The interface has been designed and implemented in 0.35 mum CMOS technology of AMI Semiconductor. The measurement result shows a noise level of less than 15 muV for a measurement time of 0.5 ms. When the measurement time is increased to 150 ms the noise level is less than 1muV.