LM134/LM234/LM334
Application Hints
(Continued)
Terminating Remote Sensor for Voltage Output
If the forward voltage drop of the diode was 0.65V instead of
the estimate of 0.6V (an error of 8%), the actual set current
will be
an error of less than 5%.
If the estimate for the tempco of the diode’s forward voltage
drop was off, the tempco cancellation is still reasonably
effective. Assume the tempco of the diode is 2.6mV/˚C in-
stead of 2.5mV/˚C (an error of 4%). The tempco of the circuit
is now:
00569714
Low Output Impedance Thermometer
A 1mA LM134 current source with no temperature compen-
sation would have a set resistor of 68Ω and a resulting
tempco of
00569706
*Output impedance of the LM134 at the “R” pin is approximately
So even if the diode’s tempco varies as much as
±
4% from
its estimated value, the circuit still eliminates 98% of the
LM134’s inherent tempco.
Typical Applications
Ground Referred Fahrenheit Thermometer
where R
2
is the equivalent external resistance connected from the V
−
pin
to ground. This negative resistance can be reduced by a factor of 5 or
more by inserting an equivalent resistor R
3
= (R
2
/16) in series with the
output.
Low Output Impedance Thermometer
00569715
*Select R3 = V
REF
/583µA. V
REF
may be any stable positive voltage
≥
2V
Trim R3 to calibrate
00569716
www.national.com
8
NSC [ NATIONAL SEMICONDUCTOR ]
