Datasheet

LM134, LM234, LM334
www.ti.com SNVS746C –MAY 2004–REVISED MARCH 2005
LEAD RESISTANCE
The sense voltage which determines operating current of the LM134 is less than 100mV. At this level,
thermocouple or lead resistance effects should be minimized by locating the current setting resistor physically
close to the device. Sockets should be avoided if possible. It takes only 0.7Ω contact resistance to reduce output
current by 1% at the 1 mA level.
SENSING TEMPERATURE
The LM134 makes an ideal remote temperature sensor because its current mode operation does not lose
accuracy over long wire runs. Output current is directly proportional to absolute temperature in degrees Kelvin,
according to the following formula:
(4)
Calibration of the LM134 is greatly simplified because of the fact that most of the initial inaccuracy is due to a
gain term (slope error) and not an offset. This means that a calibration consisting of a gain adjustment only will
trim both slope and zero at the same time. In addition, gain adjustment is a one point trim because the output of
the LM134 extrapolates to zero at 0°K, independent of R
SET
or any initial inaccuracy.
Figure 6. Gain Adjustment
This property of the LM134 is illustrated in the accompanying graph. Line abc is the sensor current before
trimming. Line a′b′c′ is the desired output. A gain trim done at T2 will move the output from b to b′ and will
simultaneously correct the slope so that the output at T1 and T3 will be correct. This gain trim can be done on
R
SET
or on the load resistor used to terminate the LM134. Slope error after trim will normally be less than ±1%.
To maintain this accuracy, however, a low temperature coefficient resistor must be used for R
SET
.
A 33 ppm/°C drift of R
SET
will give a 1% slope error because the resistor will normally see about the same
temperature variations as the LM134. Separating R
SET
from the LM134 requires 3 wires and has lead resistance
problems, so is not normally recommended. Metal film resistors with less than 20 ppm/°C drift are readily
available. Wire wound resistors may also be used where best stability is required.
APPLICATION AS A ZERO TEMPERATURE
COEFFICENT CURRENT SOURCE
Adding a diode and a resistor to the standard LM134 configuration can cancel the temperature-dependent
characteristic of the LM134. The circuit shown in Figure 7 balances the positive tempco of the LM134 (about
+0.23 mV/°C) with the negative tempco of a forward-biased silicon diode (about −2.5 mV/°C).
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