Design of mm-Sized Wirelessly Powered Sensors

This work presents several circuits that can be used to improve the range and decrease the size of millimeter-sized wirelessly powered sensors. It is shown that this can be accomplished by decreasing the power consumption of the sensor, and several circuits common to wireless sensors are presented that consume very low amounts of power, in the 100 nW range. A wireless power transmission system that can generate power in the range of 1 µW is also presented.

A circuit that allows for sensor identification is presented, which generates a unique ID number based on variation inherent to the CMOS manufacturing process. The ID generator consumes just 39 fJ/bit from a 0.4 V supply. A method of ensuring reliable identification in the presence of unreliable bits is also presented, and a system that can identify 1000 unique chips, using 31 bits per chip and with an error rate of less than 1 in 1 million is presented.

A combined demodulator and clock generator circuit is presented. Both operations are based on a low-power delay line, and it is shown that accuracy problems related to low-power operation can be mitigated by using an adaptive approach in which the delay is tuned to a transmitted reference signal. The same calibrated delay line can be used to perform demodulation and generate a 100 kHz on-chip clock, consuming 220 and 190 nW, respectively, from an 0.4 V supply. An improved version is also presented, with simulated power consumption of 130 nW during demodulation and 70 nW during clock generation.

A wireless power transmission system is also presented for a wireless sensor is also presented. Using a loop antenna formed from a bond wire, with a loop area of 12.3 mm², the system is able to transmit 2 µW over a range of 20 cm, or a minimum of 0.8 µW over a 800 cm² area, with a minimum output voltage of 0.4 V DC. The system uses a low-cost patch antenna for transmission, and could be used to cover an arbitrarily large area by tiling transmitting antennas.