Book Analog Interfacing to Embedded Microprocessors
The
formula for thermocouple voltage is a polynomial, just like thermistor resis-
tance is.
Solid State
The simplest semiconductor temperature sensor is a PN junction, such as a
signal diode or the base-emitter junction of a transistor. If the current through
the forward-biased silicon PN junction is held constant, the forward drop
decreases about 1.8 mV per °C. The Maxim MAX1617 is an IC that measures
temperature using an external transistor, such as a 2N3904, as a temperature
sensing element. The transistor can be a discrete part, or it can be embedded
in the die of an IC to measure the IC temperature. The MAX1617 has a serial
SMBus output.
The LM335 (Figure 3.5) from National Semiconductor produces an output
voltage proportional to temperature. The LM135 produces 10 mv per degree
Kelvin. At 0°C, the output is 2.73v, and at 100°C the output is 3.73v. The
LM335 operates with input current from 400 ma to 5 ma.
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