Datasheet

TL05x, TL05xA
ENHANCED-JFET LOW-OFFSET
OPERATIONAL AMPLIFIERS
SLOS178A – FEBRUARY 1997 - REVISED FEBRUARY 2003
45
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
APPLICATION INFORMATION
analog thermometer
By combining a current source that does not vary over temperature with an instrumentation amplifier, a precise
analog thermometer can be built (see Figure 89). Amplifier U1A and IC1 establish a constant current through
the temperature-sensing diode D1. For this section of the circuit to operate correctly, the TL05x must use split
supplies, and R3 must be a metal-film resistor with a low temperature coefficient.
The temperature-sensitive voltage from the diode is compared to a temperature-stable voltage reference set
by IC2. R4 should be adjusted to provide the correct output voltage when the diode is at a known temperature.
Although this potentiometer resistance varies with temperature, the divider ratio of the potentiometer remains
constant.
Amplifiers U1B, U2A, and U2B form the instrumentation amplifier that converts the difference between the diode
and reference voltage to a voltage proportional to the temperature. With switch S1 closed, the amplifier gain
equals 5 and the output voltage is proportional to temperature in degrees Celsius. With S1 open, the amplifier
gain is 9 and the output is proportional to temperature in degrees Fahrenheit. Every time S1 is changed, R4 must
be recalibrated. By setting S1 correctly, the output voltage equals 10 mV per degree (C or F).
+
–
+
–
+
–
IC1
C1
150 pF
R1
100 kΩ
U1A
R3
10 kΩ
(see Note B)
D1
(see Note A)
+15 V
R2
100 kΩ
IC2
R4
50 kΩ
U1B
R6
10 kΩ
R5
5 kΩ
R7
5 kΩ
S1
(see Note C)
R8
10 kΩ
U2A
R10
10 kΩ
R11
R9 R12
10 kΩ 10 kΩ
+15 V
+
–
–15 V
10 kΩ
V
O
(see Note D)
U2B
NOTES: A. Temperature-sensing diode ≈ (–2 mV/°C)
B. Metal-film resistor (low temperature coefficient)
C. Switch open for °F and closed for °C
D. V
O
α temperature; 10 mV/°C or 10 mV/°F
E. U1, U2 = TL05x. IC1, IC2 = LM385, LT1004, or LT1009 voltage reference
Figure 89. Analog Thermometer