A Unique Compensation Circuit
(Continued)
Thus, a current which has a square law characteristic and is
PTAT
2
, is generated for use as a means of curvature correc-
tion.
Processing and Layout
The sensor is constructed using conventional bipolar epi-
taxial linear processing. SiCr thin-film resistors are used in
place of their diffused counterparts as a result of their better
tempco matching, an important consideration for resistors
which must track over temperature. Such resistors include
R1 and nR1 of the bandgap circuit.
Another point of interest in the construction of the device
centers around transistors Q1 and Q2 of Figure 4. In order
for the circuit to retain its accuracy over temperature, the
leakage currents of each transistor, which can become quite
significant at high temperatures, must be equal so that their
effects will cancel one another. If the geometries of the two
transistors were equivalent, then their leakage currents
would be also, but since Q1 has ten times the emitter area of
Q2, the accuracy of the device could suffer. To correct the
problem, the circuit is built with Q1 and Q2 each replaced by
a transistor group consisting of both Q1 and Q2. These
transistor groups have equivalent geometries so that their
leakage currents will cancel, but only one transistor of each
group, representing Q1 in one group and Q2 in the other pair
is used in the temperature sensing circuit. A circuit diagram
demonstrating this idea is shown in Figure 8.
Using the LM34
The LM34 is a versatile device which may be used for a wide
variety of applications, including oven controllers and remote
temperature sensing. The device is easy to use (there are
only three terminals) and will be within 0.02˚F of a surface to
which it is either glued or cemented. The TO-46 package
allows the user to solder the sensor to a metal surface, but in
doing so, the GND pin will be at the same potential as that
metal. For applications where a steady reading is desired
despite small changes in temperature, the user can solder
the TO-46 package to a thermal mass. Conversely, the
thermal time constant may be decreased to speed up re-
sponse time by soldering the sensor to a small heat fin.
Fahrenheit Temperature Sensors
As mentioned earlier, the LM34 is easy to use and may be
operated with either single or dual supplies. Figure 9 shows
a simple Fahrenheit temperature sensor using a single sup-
ply. The output in this configuration is limited to positive
temperatures. The sensor can be used with a single supply
over the full −50˚F to +300˚F temperature range, as seen in
Figure 10, simply by adding a resistor from the output pin to
ground, connecting two diodes in series between the GND
pin and the circuit ground, and taking a differential reading.
This allows the LM34 to sink the necessary current required
for negative temperatures. If dual supplies are available, the
sensor may be used over the full temperature range by
merely adding a pull-down resistor from the output to the
negative supply as shown in Figure 11. The value of this
resistor should be |−V
S
|/50 µA.
For applications where the sensor has to be located quite a
distance from the readout circuitry, it is often expensive and
inconvenient to use the standard 3-wire connection. To over-
come this problem, the LM34 may be connected as a
two-wire remote temperature sensor. Two circuits to do this
are shown in Figure 12 and Figure 13. When connected as a
remote temperature sensor, the LM34 may be thought of as
a temperature-dependent current source. In both configura-
tions the current has both a relatively large value,
00905108
FIGURE 8.
AN-460
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