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Most commonly-used electrical temperature sensors are difficultto apply. For example, thermocouples have low outputlevels and require cold juncTIon compensaTIon. Thermistorsare nonlinear. In addiTIon, the outputs of these sensors arenot linearly proporTIonal to any temperature scale. Earlymonolithic sensors, such as the LM3911, LM134 and LM135,overcame many of these difficulties, but their outputs arerelated to the Kelvin temperature scale rather than the morepopular Celsius and Fahrenheit scales. Fortunately, in 1983two I.C.’s, the LM34 Precision Fahrenheit Temperature Sensorand 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 tothe LM35.The LM34 has an output of 10 mV/°F with a typical nonlinearityof only ±0.35°F over a −50 to +300°F temperaturerange, and is accurate to within ±0.4°F typically at roomtemperature (77°F). The LM34’s low output impedance andlinear output characteristic make interfacing with readout orcontrol circuitry easy. An inherent strength of the LM34 overother currently available temperature sensors is that it is notas susceptible to large errors in its output from low levelleakage currents. For instance, many monolithic temperaturesensors have an output of only 1 μA/°K. This leads to a1°K error for only 1 μ-Ampere of leakage current. On theother hand, the LM34 may be operated as a current modedevice providing 20 μA/°F of output current. The same 1 μAof leakage current will cause an error in the LM34’s output ofonly 0.05°F (or 0.03°K after scaling).
