Book Analog Interfacing to Embedded Microprocessors
there is little change in voltage with temperature.
Finally, some MOSFET transistors have a fourth lead that provides a frac-
tion of the current passed through the MOSFET itself. By connecting a resis-
tor from this lead to ground, a voltage is developed that is proportional to
the current in the device. This can be used to measure the heater resistance
when the heater is on. International Rectifier makes a line of MOSFETS,
called SENSEFETS, with this feature.
RTD heaters have some drawbacks. The first is the tolerance of the heater
element itself. Unlike RTD sensors, RTD heaters are usually sprayed or sput-
tered onto some kind of flexible substrate. Consequently, they have a typical
tolerance of about 10%, although some vendors will allow you to specify 5%
tolerance at additional cost.
Another problem with RTD heaters is that the temperature measurement
is dependent on the supply voltage
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