Simultaneous Temperature and Capacitance Sensing With Self Decoupling for Analog Pulse-Width-Modulation Backscatter

Battery-less RFID sensor tags in the Internet of Things (IoT) call for low-cost and power-efficiency multiparameter sensor design. Traditional multiparameter sensors rely on time-multiplexed parameter selection to avoid output coupling, which requires extra control logic and increases cost and design complexity. This paper presents a temperature and capacitance (T/C) sensor achieving self decoupling through the proposed self-switching double sampling (SDS) interface. With double sampling, a temperature-sensitive current alternately charges a reference capacitor and a sensing capacitor, simultaneously translating T/C information into a pulse-width-modulated (PWM) waveform. Measuring the low pulse width and the pulse width ratio can decouple temperature and capacitance information, respectively, eliminating the demand for parameter selection. Meanwhile, SDS reuses the PWM waveform as the doublesampling control signal without external control logic. The PWM signal can be sent back by analog PWM backscatter without the need for digitization. The SDS sensor applies a designed dualslope relaxation oscillator (RxO) with area efficiency, ultralow power, and inherent self-switching topology as the T/C-to-PWM converter. A prototype sensor has been designed and simulated in <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$55-\text{nm}$</tex> CMOS technology, occupying <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$0.037 ~\text{mm}^{2}$</tex> and consuming 59.6 nW at room temperature with a <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$0.8-\mathrm{V}$</tex> supply voltage. The T/C sensor achieves a temperature inaccuracy of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$-0.58 /+0.38{ }^{\circ} \mathrm{C}$</tex> in <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$-20 \sim 120{ }^{\circ} \mathrm{C}$</tex> and a capacitance inaccuracy of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$-24.9 /+20 \text{fF}$</tex> in <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$0 \sim 30 \text{pF}$</tex>.