Analysis And Design Of CMOS Temperature Sensor For RFID Tags

As an important kind of sensor, the temperature sensor takes up more than40%of thetotal demand of the sensors. It is a kind of semiconductor devices that can transfer thenon-electrical physical quantities into electrical quantities by taking advantage that thephysical parameters of the material and component can be vary with temperature so that itcan do accurate temperature measurement and automatic control. RFID (Radio FrequencyIdentification) is a kind of non-contact automatic identification technology, which uses RFsignals to automatically identify the target object and acquire related data. This automaticidentification does not need any human intervention and can be able to work in any hashenvironments. The RFID tag chip with integrated temperature sensor can monitor theambient temperature at any time and obtain data without delay. These properties offer it abroader application prospect in the fields of medical treatment, product production andsupply-chain management, and it is currently the hotspot in international academia andindustry as well.This paper describes a high accuracy CMOS temperature sensor that integrated in theRFID tag chip. This temperature sensor has the features of low power consumption andhigh accuracy and can be used for accurate measurement of the ambient temperature. Thetemperature sensor designed in this paper includes two parts: PTAT (Proportional ToAbsolute Temperature) current generation circuit and CCO (Current-Controlled Oscillator).PTAT current generating circuit utilizes the temperature feature of the MOSFET whichworks at the subthreshold region current, to generate the PTAT current. This strategyreplaces the traditional bandgap circuit. CCO transfers the PTAT current into the periodicsquare wave signal which proportional to frequency and temperature. This square wavesignal will change into digital signal through a counter.In this paper, the CMOS temperature sensor is designed by using the software Virtuosoof Cadence. The Spectre is used to do simulation on every module of the temperature sensor and the whole circuit. The SMIC0.18μm2P4M CMOS process model is adopted inall device model parameters. When the working volt of the temperature sensor is1.8V, theworking current is no larger than50μA. When the measurable temperature ranges from–20℃to50℃, the minimum resolution is75LSB/℃, the linearity error is-0.1/+0.3℃. The rate ofchange of the PTAT current is0.188nA/℃. The output current of the current referencesource is180nA, the temperature is-20.0℃, the reference source current is181.4nA, whenthe temperature is37.0℃the reference source current is182.06nA, when the temperatureis50.0℃the reference source current is182.02nA.