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RFID tag performance optimization: a chip perspective 45
change depending on the detected temperature. A digital temperature sensor can be
considered as an analog temperature sensor with integrated ADC. It provides
digital temperature information. In an analog temperature sensor, the sensor gain is
Sensor gain ¼
Supply voltage
Temperature range
ð3:6Þ
According to this equation, if the power supply is 1.8 V and the temperature range
is 40 Ctoþ 80 C, the sensor gain is 15 mV/ C. The lower the supply voltage,
the smaller the sensor gain. How to handle small gains and monitor the temperature
accurately is the most difficult problem in integrated temperature sensor
design. Even for a digital temperature sensor implementing low-power highaccuracy
ADC is also a big challenge.
Challenges of temperature sensor in an RFID chip
Our objective has been to review requirements to integrate temperature sensor and
RFID tag functionality into a chip. The requirement constraints of RFID tag chip
design are equally important for the temperature sensor, such as ultra-low power,
small area, low cost, and high performance. At the same time, error cancellation
and temperature calibration in a sensor should be implemented at a low cost.
We can summarize the key techniques involved in a temperature sensor
according to the following sample digital temperature sensor design steps. The
block scheme is shown in Fig. 3.9.
1. The control block is needed to control the power-down mode and the counter.
The RD converters have been proven to be very suitable in low-frequency, highperformance
applications.
2. The bit stream at the output is directly proportional to I temp/Iref; then the signal
is led to a counter and the output signal becomes an 8-bit code.
3. Before the signal out of the counter can be read on the monitor as representing
temperature, the sensor must be calibrated. The calibration figures are stored
in an EEPROM in the other calibration facility; the relation of the 8-bit code
and the temperature can be fixed once and for all by means of actual
measurement.
4. In the temperature sensor and reference generator block, a PTAT circuit with
chopping technique is a good solution to generate the linear function of
temperature.
5. Offset cancellation techniques, such as the use of a nested-chopper amplifier,
should be embedded within the temperature sensor system.
6. This also reduces the errors caused by the chip’s self-heating, so that accurate
detected temperatures can be shown.
This chapter has covered the metrics, performance characteristics, and chip
layout constraints of integrating sensors on a tag. Starting with this analysis of