3. Serial-output temperature Sensors. Typically, serial-output temperature sensors use a two- or three-wire interface to the host microcontroller. These devices have an integrated ADC that converts the analog output of the internal sensing element to a digital output. They can achieve temperature accuracies as high as 0.5 degrees Celsius , with a measurement resolution of less than 0.1 degrees Celsius .
Many serial-output temperature sensors provide user-programmable functions, such as over- and under-temperature alerts and integrated EEPROM for general-purpose data storage. These features can be used to simplify a design, increase design flexibility, improve temperature sensing accuracy, and lower overall system cost. The over- and under-temperature alert feature works in the same way as it does for logic-output temperature sensors.
Using the serial interface, the host MCU loads temperature trigger limits in degrees Celsius, into an internal register located in the silicon temperature sensor. When the desired temperature limit is exceeded, the sensor flags the host controller that an over or under-temperature condition occurred. This feature can be used to turn on a light or control a fan via a serial interface, without the need for the microcontroller to monitor temperature continuously.
This increases flexibility by freeing up the host microcontroller from having to continuously monitor the system. It also simplifies software and hardware development.
Many applications today require temperature accuracy of less than 0.5 degrees Celsius , over a fairly wide temperature range. Higher accuracy can be achieved with many silicon-based temperature sensors via a calibration lookup table that calibrates the sensor at various known temperatures.
The number of calibration points depends on the temperature range, the accuracy required and the non-ideal characteristics of the sensor. The accuracy vs. temperature graph in Figure 6 below demonstrates how a typical silicon temperature sensor's accuracy varies over temperature, before and after non-ideality error compensation.
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| Figure 6: Shown is silicon IC accuracy vs. temperature characteristics with and without compensation. |
Figure 6 shows sensor accuracy before and after error compensation. The temperature sensor's non-ideality characteristics are illustrated in a predictable curve shape and can be described using a second-order polynomial equation.
The polynomial equation coefficients are generated by taking multiple temperature error points, from maximum to minimum temperature limits. The equation can be used to compensate the sensor temperature error by computing the equation at the measured temperature using a microcontroller.
The equation can also be used to generate a look up table that can be stored in EEPROM. Some temperature-sensor ICs have integrated 256bytes of EEPROM, which can be used to store the sensor non-ideality characteristics in a look-up table. The on-chip EEPROM can also be used for general-purpose data storage.
Temperature sensors bring with them a variety of advantages and disadvantages. No one type of sensor is appropriate for all temperature-sensing applications. To identify the appropriate sensor, the specific requirements of each application must be outlined.
Thermistors provide a useful, low-cost temperature-sensing solution for applications that operate over a limited temperature range due to its nonlinear characteristics. RTDs can be highly accurate over a several 100s of degrees Celsius range. RTDs require high-performance instrumentation systems that require manual scaling and calibration, which makes them more costly.
Thermocouples are most useful in applications that must operate in temperature extremes of less than -200 or more than 1,000 degrees Celsius . These sensors require high-performance instrumentation systems, which can be very costly. On the other hand, silicon IC-based temperature sensors simplify designs, while offering relatively high accuracy over a temperature range of -55 to 150 degrees Celsius . They also provide many integrated features that enhance system flexibility and performance.
( John Austin is Principal Product Marketing Engineer and Ezana Haile is Senior Applications Engineer in the Analog & Interface Products Division at Microchip Technology Inc.)
