Relaxation oscillator based CMOS temperature sensor with capacitor flipping
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- Relaxation oscillator based CMOS temperature sensor with capacitor flipping
- Other Titles
- Capacitor flipping 기술이 추가되어 Relaxation oscillator를 기반으로 한 CMOS 온도 센서
- Shin, Changyong
- Kim, Seong-Jin
- temperature sensor; relaxation oscillator; offset cancellation
- Issue Date
- Graduate School of UNIST
- Temperaturecan be a factor that affect anykind ofsystem so measuring and predicting temperature is necessaryfor stablesystem. In bio-signal acquisition or artificial viscera, body is vulnerable to heat, so monitoring temperature is necessary. Not only human’s body but also electronicdevice and chip are vulnerable to heat. For overheating protection applications, temperature must be monitored. In environmental and industrial system, temperature is important factor. For better performance and preventing accident, temperature must be adjusted stably. If temperature changes suddenly, we must observe it and adjust temperature properly.
Method for measuring temperature is so various.In real life, people mostly use mercury thermometer.The characteristic of temperature dependence is utilized. However, it is hard to communicate with the electronic device. That’s why we need smart temperature sensor. Temperature sensor needs to be integrated in electronic device.
Most of the passive devices show temperature dependency. Using that characteristic, electronic smart temperature sensor is invented. Resistors, BJTs, MOSFETs, diodes, are the examples. It has its pros and cons so selection should be considered smartly. Resistor is power effective, but it wastes area, and temperature dependency is poor so multi-point trimming is necessary. MOSFET is power effective, but its temperature dependency is poor, so we need additional technique that can compensate non-linearity. However, BJT still shows poor temperature dependency.
If we observe the base-emittervoltage with change of temperature, curvature is observed. Nevertheless, it shows betterdependencythan othersGenerally, BJT is widely utilized as smart temperature sensor because of its linearity. However, it shows better performance relatively with other passive devices, but it still lacks accuracy and dependency. That’s why solution for reducing non-linearity is significant. Trimming or calibration technique is needed for that problem.In circuit, trimming can be done with adjusting biasing current or voltage. However, it can be a burden and the production cost increases. Today’s trend does calibration or trimming at the back stage digitally. It gives simplicity and cost effective.
Not only the sensing type but also analog circuit itself can be errorsource. In the layout, mismatch of the devices and the parasitic capacitance can degrade the performance of circuit Amplifier and comparator can be affected by offset. Mismatch of the current mirror can give the incorrect bias current ratio. To reducethese errors, many efforts are done. Many techniques are utilized in the circuit. Chopping can mitigate the error caused by offset. DEM(Dynamic Element Matching) can reduce the error of current source mismatch. Various techniques are included in this CMOS smart temperature sensor.
Readout circuit as well as bias circuit is significant. Input type can be current, voltage, time, frequency. Readout circuit can also be many kinds of circuits. ADC(Analog-to-digital converter), FDC(Frequency-to-digital converter), TDC(Time-to-digital converter) can be connected to get a output. Each readout circuits include their own pros and cons, so we must choose the readout circuit with our expected spec.
Calibration and trimming can be important factor in CMOS temperature sensor. It has direct relationship with the production cost. The more calibration and trimming is needed, the much money is payed. Effort to reduce calibration point or simplifying trimming is in progress.
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