A 0.037-mm², 65.8-nW Temperature and Capacitance Sensor With Analog Pulse-Width-Modulation Backscatter

Battery-less RFID sensor tags in the Internet of Things (IoT) expect low-cost and power-efficiency multiparameter sensing solutions. Traditional sensor designs rely on time-multiplexed parameter selection to prevent output coupling, which introduces extra control logic and increases cost and design complexity. This paper presents a temperature and capacitance (T/C) sensor with analog pulse-width-modulated (PWM) backscatter. The sensor achieves self-decoupling T/C sensing through the proposed self-switching double sampling (SDS) interface, eliminating the demand for parameter selection. With double sampling, a temperature-sensitive current alternately charges a reference capacitor and a sensing capacitor, simultaneously translating T/C information into a PWM waveform. The low pulse width (LPW) and pulse width ratio (PWR) independently represent temperature and capacitance, enabling simultaneous and decoupled readout. Meanwhile, SDS reuses the PWM waveform as the double-sampling control signal without external control logic. The PWM signal is sent back by analog PWM backscatter without the need for digitization. The SDS sensor employs a compact, ultra-low-power dual-slope relaxation oscillator (RxO) with inherent self-switching topology for T/C-to-PWM conversion. Fabricated in 55-nm CMOS technology, the sensor occupies 0.037 mm2 and consumes 65.8 nW at 0.8 V. Measurement results show that the T/C sensor achieves a temperature inaccuracy of −1.22/+1.17°C ( $3{\sigma }$ ) in $- 20\sim 100^{\circ }$ C and a capacitance inaccuracy of −197/192 fF ( $3{\sigma }$ ) in $0\sim 35$ pF.