An integrated controller for tunnel sensors

Micromachined tunnel sensors require electronic circuitry to servo control their tunnel gap width. In this paper, we present the first integrated and complete tunnel sensor controller. This self-contained CMOS controller provides all necessary controller functions; it generates a tunnel junction bias, senses the tunnel current by dropping it across a diode-connected transistor, compares the voltage across this diode to an internally generated proportional to-absolute-temperature reference, derives a feedback signal with an externally configurable frequency response, generates a high-voltage electrostatic drive, and provides a low-voltage output for external circuitry. The chip requires -5 and -40-V power supplies and a compensation RC network, consumes 0.9 mW power, has a power supply rejection ratio of 30 dB, and has a total die area, exclusive of bond pads, of 0.35 mm/sup 2/. The circuit compensates for tunnel junction nonlinearity and contributes less signal noise than the sensor that it controls.