OPA1013A1

R2

R1

VO

MotorolaMTS102

100A100A

REF200

4.5V to 36VDiodes are frequently used as temperature sensors in a widevariety of moderate-precision temperature measurementapplications. The relatively high temperature coefficient ofabout 2mV/C is fairly linear. To make a temperaturemeasurement 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 avariety of applications.

THE DIODEJust about any silicon diode can be used as a temperaturemeasurement transducer. But the Motorola MTS102 SiliconTemperature Sensor is a diode specifically designed andoptimized for this function. It is intended for temperaturesensing applications in automotive, consumer and industrialproducts where low cost and high accuracy are important.Packaged in a TO-92 package it features precise temperatureaccuracy of 2C from 40C to +150C.

EXCITATIONA current source is the best means for diode excitation. Insome instances, resistor biasing can provide an adequateapproximation, but power supply variations and ripple cancause significant errors with this approach. These problemsare exacerbated in applications with low power supplyvoltages such as 5V single supply systems. Since the MTS102is specified for 100A operation, the Burr-Brown REF200Dual 100A Current Source/Sink makes the perfect match.One current source can be used for excitation and the othercurrent source can be used for offsetting.

AMPLIFICATIONIn most instances, any precision op amp can be used fordiode signal conditioning. Speed is usually not a concern.When 15V supplies are available, the low cost precisionOPA177 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 powersupply rail (ground in single-supply applications), and itsoutput can swing to within about 15mV of the negative rail.Figure 1 shows the simplest diode-based temperature mea-surement system. One of the 100A current sources in theREF200 is used for diode excitation. The other currentsource is used for offsetting. One disadvantage of this circuitis that the span (GAIN) and zero (OFFSET) adjustments areinteractive. You must either accept the initial errors or use an

VO = VBE (1 + R2/R1) 100A R2Where:VBE = voltage across diode (V)Zero and span adjustments withR1 and R2 are interactive.

interactive adjustment technique. Another possible disad-vantage is that the temperature to voltage conversion isinverting. In other words, a positive change in temperatureresults in a negative change in output voltage. If the outputis to be processed in a digital system, neither of theselimitations may be a disadvantage.The following relationships can be used to calculate nominalresistor values for the Figure 1 circuit.

BASIC TRANSFER FUNCTIONVO = VBE (1 + R2/R1) 100A R2

CALCULATING RESISTOR VALUES

R1 =

R2 = R1 ( 1)

Where:R1, R2 = Resistor values ()VBE = Voltage across diode (V)VBE25 = Diode voltage at 25C (V)Three choices are available for the MTS102See tableon page 2.

(VO/T) (VBE25 + TC (TMIN 25C)) (TC V1)100A ((VO/T) TC)

(VO/T)TC

Figure 1. Simple Diode-based Temperature MeasurementCircuit.

APPLICATION BULLETINMailing Address: PO Box 11400 Tucson, AZ 85734 Street Address: 6730 S. Tucson Blvd. Tucson, AZ 85706Tel: (602) 746-1111 Twx: 910-952-111 Telex: 066-6491 FAX (602) 889-1510 Immediate Product Info: (800) 548-6132

DIODE-BASED TEMPERATURE MEASUREMENTBY R. MARK STITT AND DAVID KUNST (602) 746-7445

1991 Burr-Brown Corporation AB-036 Printed in U.S.A. September, 1991

SBOA019

2V1 = Output voltage of circuit at TMIN (V)VO = Output voltage of circuit (V)TC = Diode temperature coefficient (V/C)

TC value depends on VBE25See table below.TMIN = Minimum process temperature (C)VO/T = Desired output voltage change for given

temperature change (V/C)(Note: Must be negative for Figure 1 circuit.)

AVAILABLE VBE25 AND TC VALUES FORMOTOROLA MTS102 TEMPERATURE SENSOR

EXAMPLEDesign a temperature measurement system with a 0 to 1.0Voutput for a 0 to 100C temperature.TMIN = 0C

VO/T = (1V 0V)/(100C 0C) = 0.01V/C

If VBE25 = 0.595V, TC = 0.002265V/C, andR1= 8.424kR2= 28.77k

For a 0 to 10V output with a 0 to 100C temperature:R1= 6.667kR2= 287.7kIf independent adjustment of offset and span is requiredconsider the circuit shown in Figure 2. In this circuit, a thirdresistor, RZERO is added in series with the temperature-sensing diode. System zero (offset) can be adjusted withRZERO without affecting span (gain). To trim the circuit adjustspan first. Either R1 or R2 (or both) can be used to adjustspan. As with the Figure 1 circuit this circuit has the possibledisadvantage that the temperature to voltage conversion isinverting.The following relationships can be used to calculate nominalresistor values for the Figure 2 circuit.

BASIC TRANSFER FUNCTIONVO = (VBE + 100A RZERO) (1 + R2/R1) 100A R2

CALCULATING RESISTOR VALUESSet RZERO = 1k (or use a 2k pot)

R1 =

VBE25 TC(V) (V/C)

0.580 0.0023150.595 0.0022650.620 0.002183

(VO/T) (VBE25 + (RZERO 100A) + TC (TMIN 25C)) (TC V1)100A ((VO/T) TC)

(VO/T)TC

R2 = R1 ( 1)

Where:RZERO = Zero (offset) adjust resistor ()Others = as before

EXAMPLEDesign a temperature measurement system with a 0 to 1.0Voutput for a 0 to 100C temperature.TMIN = 0C

VO/T = (1V 0V)/(100C 0C) = 0.01V/C

If VBE25 = 0.595V, TC = 0.002265V/C, andRZERO = 1k (use 2k pot)R1 = 9.717kR2 = 33.18k

For a 0 to 10V output with a 0 to 100C temperature:RZERO = 1k (use 2k pot)R1 = 7.69kR2 = 331.8k

Figure 2. Diode-based Temperature Measurement Circuitwith Independent Span (gain) and Zero (offset)Adjustment.

VO = (VBE + 100A RZERO) (1 + R2/R1) 100A R2Where:VBE = voltage across diode (V)Adjust span first with R1 or R2 then adjust zero with RZEROfor noninteractive trim.

OPA1013A1

R2

VO

MotorolaMTS102

100A100A

REF200

4.5V to 36V

RZero R1

3OPA1013A1

R2

VOMotorolaMTS102

100A100A

REF200

VS

RZeroR1

For a noninverting temperature to voltage conversion, con-sider the circuit shown in Figure 3. This circuit is basicallythe same as the Figure 2 circuit except that the amplifier isconnected to the low side of the diode. With this connection,the temperature to voltage conversion is noninverting. Asbefore, if adjustment is required, adjust span with R1 or R2first, then adjust zero with RZERO.A disadvantage of the Figure 3 circuit is that it requires anegative power supply.The following relationships can be used to calculate nominalresistor values for the Figure 3 circuit.

BASIC TRANSFER FUNCTIONVO = (VBE 100A RZERO) (1 + R2/R1) + 100A R2

CALCULATING RESISTOR VALUESR1 = same as Figure 2R2 = same as Figure 2Where:Components = as before

EXAMPLEDesign a temperature measurement system with a 0 to 1.0Voutput for a 0 to 100C temperature.TMIN = 0C

VO/T = (1V 0V)/(100C 0C) = 0.01V/C

If VBE25 = 0.595V, TC = 0.002265V/C, andRZERO = 1kR1 = 9.717kR2 = 33.18k

For a 0 to 10V output with a 0 to 100C temperature:RZERO = 1kR1 = 7.69kR2 = 331.8k

For a single-supply noninverting temperature to voltageconversion, consider the Figure 4 circuit. This circuit issimilar to the Figure 2 circuit, except that the temperature-sensing diode is connected to the inverting input of theamplifier and the offsetting network is connected to thenoninverting input. To prevent sensor loading, a secondamplifier is connected as a buffer between the temp sensorand the amplifier. If adjustment is required, adjust span withR1 or R2 first, then adjust zero with RZERO.The following relationships can be used to calculate nominalresistor values for the Figure 4 circuit.

Figure 3. Positive Transfer Function Temperature Measure-ment Circuit with Independent Span (gain) andZero (offset) Adjustment.

VO = (VBE 100A RZERO) (1 + R2/R1) + 100A R2Where:VBE = voltage across diode (V)Adjust span first with R1 or R2 then adjust zero with RZEROfor noninteractive trim.

BASIC TRANSFER FUNCTIONVO = 100A RZERO (1 + R2/R1) VBE R2/R1

CALCULATING RESISTOR VALUES

RZERO =

R1 = 10k (arbitrary)

R2 = R1 ( )

Where:Components = as before

EXAMPLEDesign a temperature measurement system with a 0 to 1.0Voutput for a 0 to 100C temperature.

TMIN = 0C

VO/T = (1V 0V)/(100C 0C) = 0.01V/C

If VBE25 = 0.595V, TC = 0.002265V/C, andRZERO = 5.313kR1= 10.0kR2= 44.15k

100A (TC (VO/T))(TC V1) (VO/T) (VBE25 + TC (TMIN 25C))

(VO/T)TC

4For a 0 to 10V output with a 0 to 100C temperature:RZERO = 6.372kR1 = 10.0kR2 = 441.5k

For differential temperature measurement, use the circuitshown in Figure 5. In this circuit, the differential outputbetween two temperature sensing diodes is amplified by atwo-op-amp instrumentation amplifier (IA). The IA is formedfrom the two op amps in a dual OPA1013 and resistors R1,R2, R3, R4, and RSPAN. RSPAN sets the gain of the IA. For goodcommon-mode rejection, R1, R2, R3, and R4 must be matched.If 1% resistors are used, CMR will be greater than 70dB forgains over 50V/V. Span and zero can be adjusted in anyorder in this circuit.

The following relationships can be used to calculate nominalresistor values for the Figure 5 circuit.

BASIC TRANSFER FUNCTIONVO = ((VBE2 + 100A RZERO2) (VBE1 + 100A RZERO1)) GAIN

Where:GAIN = 2 + 2 R1/RSPAN

CALCULATING RESISTOR VALUES

RSPAN =

RZERO1 = RZERO2 = 500 (use 1k pot for RZERO)

Where:RSPAN = Span (gain) adjust resistor []Others = as before

EXAMPLEDesign a temperature measurement system with a 0 to 1.0Voutput for a 0 to 1C temperature differential.TMIN = 0CVO/T = (1V 0V)/(1C 0C) = 1.0V/CIf VBE25 = 0.595V, TC = 0.002265V/C, andRZERO = 1k potR1, R2, R3, R4 = 100k, 1%RSPAN = 455

For a 0 to 10V output with a 0 to 1C temperature differen-tial:RZERO = 1k potR1, R2, R3, R4 = 100k, 1%RSPAN = 45.3

(VO/T) + 2 TC

VO = 100A RZER O (1 + R2/R1) VBE R2/R1Where:VBE = voltage across diode [V]Adjust span first with R1 or R2 then adjust zerowith RZERO for noninteractive trim.

Figure 4. Single-supply Positive Transfer Function Tempera-ture Measurement Circuit with Independent Span(gain) and Zero (offset) Adjustment.

2 R1 TC

OPA1013A1

RZERO

MotorolaMTS102

100A100A

REF200

4.5V to 36V

R2

OPA1013A2

VO

R1

5MotorolaMTS102

OPA1013A1

RSPAN

RZERO

MotorolaMTS102

100A100A

REF200

4.5V to 36V

OPA1013A2

VO

R1100k

R2100k

R3100k

R4100k

VO = ((VBE2 + 100A RZERO2) (VBE1 + 100A RZERO1)) GAINGAIN

= 2 + 2 R1/RSPANAdjust zero and span in any order.

Figure 5. Differential Temperature Measurement Circuit.

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