文档介绍：M4L3 rcso ooihcTmeaueSnosAN-460 Sensors Temperature Monolithic Precision LM34/LM35
National Semiconductor
LM34/LM35 Application Note 460
Precision Monolithic October 1986
Temperature Sensors
Introduction Another approach has been developed where the difference
in the base-emitter voltage of two transistors operated at
monly-used electrical temperature sensors are dif- different current densities is used as a measure of tempera-
ficult to apply. For example, thermocouples have low output ture. It can be shown that when two transistors, Q1 and Q2,
levels and require cold pensation. Thermistors are operated at different emitter current densities, the differ-
are nonlinear. In addition, the outputs of these sensors are ∆
ence in their base-emitter voltages, VBE,is
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 (1)
two .’s, the LM34 Precision Fahrenheit Temperature Sen- where k is Boltzman’s constant, q is the charge on an
sor and the LM35 Precision Celsius Temperature Sensor, electron, T is absolute temperature in degrees Kelvin and
were introduced. This application note will discuss the LM34, JE1 and JE2 are the emitter current densities of Q1 and Q2
but with the proper scaling factors can easily be adapted to respectively. A circuit realizing this function is shown in Fig-
the LM35. ure 1.
The LM34 has an output of 10 mV/˚F with a typical nonlin-
earity of only ±˚F over a −50 to +300˚F temperature
range, and is accurate to within ±˚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 temper