Book Analog Interfacing to Embedded Microprocessors
until the system heats up.
What I did was add a second opamp, wired as a buffer (no gain, no scaling)
and connect the input to the thermistor. The output went to a second ADC
channel. If the thermistor opens, voltage V1 will go to the reference voltage,
2.5V. In this system, 2.5V corresponds to a temperature below 0°C, outside
the allowable range of operation for the instrument. So the microprocessor
used the scaled/amplified signal to measure temperature, and monitored the
unscaled signal for a voltage greater than about 2V to detect an open ther-
mistor.
If you don’t have a second ADC channel, the same thing can be imple-
mented with a comparator. In this case, one side of the comparator would
connect to the thermistor and the other side would connect to a 2V refer-
ence. The output of the comparator then goes to a digital input that can be
monitored by the microprocessor.
RTD Heater
The RTD heater is a special type of heater that is composed of an RTD
annaabi.ee 
