Q
I need more information about the TEMP Sensor of the ADuC812. What I need is the temperature dependance between exteranal temperature and the internal temperature. If I want to messure the external temperature how can I calculate it from the value I recieve after AD conversion. Thanks. Yours Dan G.
A
As with any semiconductor temperature sensor the ADuC812 temp sensor measures the on chip die temperature (i.e the silicon junction temperature) and not the ambient temperature directly. The die temperature consist of two components, temperature due to ambient and temperature due to the chip dissipating power as heat through the thermal resistance of the package. If the ADuC812 is unpowered then the die temperature will settle to ambient temperature. Now if the ADuC812 is powered it will start to dissipate power as heat. The heat will flow from the die to the environment via the package thermal resistance (quoted as thetaJC on the datasheet) and there will be a thermal difference between die and ambient equal to: Power Dissipation x thetaJC Using this information you can calculate the ambient temperature based on the measured die temperature and the known power dissipation of the ADuC812. For example, if the ADuC812 is drawing 30mA from a 5V supply, the power dissipation is 150mW. Assuming a thermal resistance of 90degC/Watt, the temperature difference between chip and ambient will be 13.5degrees. And you can simply subtract 13.5 from the measured die temperature. The power dissipated by the ADuC812 will depend on your specific application, current consumption under different operating conditions is given in the datasheet. If power is likely to be constant then a single point calibration be all that is required. If power consumption is likely to vary then you may consider measuring the current of the ADuC812 via one of the ADC channels. Note: The temperature sensor is inherently very linear (to about +/-2deg) but it is not trimmed for absolute accuracy. We do not test the accuracy of the temp sensor and therefore do not provide guaranteed specs. Typically the output of the temp sensor is 650mV at room temperature with a tempco of -3.5mV/degC. To make accurate absolute temperature measurements, you need to perform a two point calibration and store the offset and gain co-efficients. Many designers are using the temp sensor to determine when the ambient temperature has changed "significantly" and using this to compensate a sensor for temperature drift. If you require an accurate "off the shelf " temperature sensor, then you may consider the AD7814 and AD7414, SPI and I2C temperature sensors respectively which can be simply hooked onto the serial bus of the ADuC812.