A Low-Power Temperature Sensing System For Implantable Biomedical Applications

This paper presents a low-power system that senses ambient temperature and wirelessly transmits the sensed information to a nearby receiver. Although intended for an implantable smart coronary stent, it can be used for other sensory applications that require similar temperature sensing. A thermal sensor is presented that utilizes MOSFETs biased in sub-threshold along with a combination of p(+)-poly and n(+)-poly resistors to generate a PTAT (proportional to absolute temperature) source that can operate at low supply voltages. Leveraging central limit theorem, the output currents of several PTAT sources are combined to further reduce the error. The resulting PTAT source is then used to implement a temperature controlled oscillator for frequency modulation and transmission. A prototype in 65-nm CMOS can sense and transmit temperature values in the range of 30 to 50 degrees C with an average resolution of 1.1 MHz/degrees C. The sensor-transmitter system consumes 100 mu W of DC power and delivers -24 dBm of power to a 50-Omega load.