ADuC812: Measure ambient temperature

Document created by analog-archivist Employee on Feb 23, 2016
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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.


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.