Most commonly-used electrical temperature sensors are difficult
to apply. For example, thermocouples have low output
levels and require cold junction compensation. Thermistors
are nonlinear. In addition, the outputs of these sensors are
not linearly proportional to any temperature scale. Early
monolithic sensors, such as the LM3911, LM134 and LM135,
overcame many of these difficulties, but their outputs are
related to the Kelvin temperature scale rather than the more
popular Celsius and Fahrenheit scales. Fortunately, in 1983
two I.C.’s, the LM34 Precision Fahrenheit Temperature Sensor
and the LM35 Precision Celsius Temperature Sensor,
were introduced. This application note will discuss the LM34,
but with the proper scaling factors can easily be adapted to
the LM35.
The LM34 has an output of 10 mV/°F with a typical nonlinearity
of only ±0.35°F over a ?50 to +300°F temperature
range, and is accurate to within ±0.4°F typically at room
temperature (77°F). The LM34’s low output impedance and
linear output characteristic make interfacing with readout or
control circuitry easy. An inherent strength of the LM34 over
other currently available temperature sensors is that it is not
as susceptible to large errors in its output from low level
leakage currents. For instance, many monolithic temperature
sensors have an output of only 1 μA/°K. This leads to a
1°K error for only 1 μ-Ampere of leakage current. On the
other hand, the LM34 may be operated as a current mode
device providing 20 μA/°F of output current. The same 1 μA
of leakage current will cause an error in the LM34’s output of
only 0.05°F (or 0.03°K after scaling).