廬
APPLICATION BULLETIN
BY R. MARK STITT AND DAVID KUNST (602) 746-7445
Mailing Address: PO Box 11400 鈥?Tucson, AZ 85734 鈥?Street Address: 6730 S. Tucson Blvd. 鈥?Tucson, AZ 85706
Tel: (602) 746-1111 鈥?Twx: 910-952-111 鈥?Telex: 066-6491 鈥?FAX (602) 889-1510 鈥?Immediate Product Info: (800) 548-6132
DIODE-BASED TEMPERATURE MEASUREMENT
Diodes are frequently used as temperature sensors in a wide
variety of moderate-precision temperature measurement
applications. The relatively high temperature coefficient of
about 鈥?mV/擄C is fairly linear. To make a temperature
measurement system with a diode requires excitation, offset-
ting, and amplification. The circuitry can be quite simple.
This Bulletin contains a collection of circuits to address a
variety of applications.
THE DIODE
Just about any silicon diode can be used as a temperature
measurement transducer. But the Motorola MTS102 Silicon
Temperature Sensor is a diode specifically designed and
optimized for this function. It is intended for temperature
sensing applications in automotive, consumer and industrial
products where low cost and high accuracy are important.
Packaged in a TO-92 package it features precise temperature
accuracy of
鹵2擄C
from 鈥?0擄C to +150擄C.
EXCITATION
A current source is the best means for diode excitation. In
some instances, resistor biasing can provide an adequate
approximation, but power supply variations and ripple can
cause significant errors with this approach. These problems
are exacerbated in applications with low power supply
voltages such as 5V single supply systems. Since the MTS102
is specified for 100碌A(chǔ) operation, the Burr-Brown REF200
Dual 100碌A(chǔ) Current Source/Sink makes the perfect match.
One current source can be used for excitation and the other
current source can be used for offsetting.
AMPLIFICATION
In most instances, any precision op amp can be used for
diode signal conditioning. Speed is usually not a concern.
When
鹵15V
supplies are available, the low cost precision
OPA177 is recommended. For 5V single-supply applica-
tions, the OPA1013 Dual Single-Supply op amp is recom-
mended. Its inputs can common-mode to its negative power
supply rail (ground in single-supply applications), and its
output can swing to within about 15mV of the negative rail.
Figure 1 shows the simplest diode-based temperature mea-
surement system. One of the 100碌A(chǔ) current sources in the
REF200 is used for diode excitation. The other current
source is used for offsetting. One disadvantage of this circuit
is that the span (GAIN) and zero (OFFSET) adjustments are
interactive. You must either accept the initial errors or use an
漏
4.5V to 36V
REF200
100碌A(chǔ)
100碌A(chǔ)
R
2
A
1
OPA1013
V
O
R
1
Motorola
MTS102
V
O
= V
BE
(1 + R
2
/R
1
) 鈥?100碌A(chǔ) 鈥?R
2
Where:
V
BE
= voltage across diode (V)
Zero and span adjustments with
R
1
and R
2
are interactive.
Figure 1. Simple Diode-based Temperature Measurement
Circuit.
interactive adjustment technique. Another
possible
disad-
vantage is that the temperature to voltage conversion is
inverting. In other words, a positive change in temperature
results in a negative change in output voltage. If the output
is to be processed in a digital system, neither of these
limitations may be a disadvantage.
The following relationships can be used to calculate nominal
resistor values for the Figure 1 circuit.
BASIC TRANSFER FUNCTION
V
O
= V
BE
(1 + R
2
/R
1
) 鈥?100碌A(chǔ) 鈥?R
2
CALCULATING RESISTOR VALUES
R
1
= (未V /未T) 鈥?(V
+ T
C
鈥?(T
MIN
鈥?5擄C)) 鈥?(T
C
鈥?V1)
O
BE25
100碌A(chǔ) 鈥?((未V
O
/未T) 鈥?T
C
)
R
2
= R
1
鈥?(
Where:
R
1
, R
2
= Resistor values (鈩?
V
BE
= Voltage across diode (V)
V
BE25
= Diode voltage at 25擄C (V)
Three choices are available for the MTS102鈥擲ee table
on page 2.
Printed in U.S.A. September, 1991
(未V
O
/未T)
T
C
鈥?1)
1991 Burr-Brown Corporation
AB-036
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2
3
4
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