APPLICATION BULLETIN ® Mailing Address: PO Box 11400 • Tucson, AZ 85734 • Street Address: 6730 S. Tucson Blvd. • Tucson, AZ 85706 Tel: (602) 746-1111 • Twx: 910-952-111 • Telex: 066-6491 • FAX (602) 889-1510 • Immediate Product Info: (800) 548-6132 DIODE-BASED TEMPERATURE MEASUREMENT BY R. MARK STITT AND DAVID KUNST (602) 746-7445 Diodes are frequently used as temperature sensors in a wide variety of moderate-precision temperature measurement applications. The relatively high temperature coefficient of about –2mV/°C is fairly linear. To make a temperature measurement system with a diode requires excitation, offsetting, and amplification. The circuitry can be quite simple. This Bulletin contains a collection of circuits to address a variety of applications. 4.5V to 36V REF200 100µA 100µA R2 A1 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 ±2°C from –40°C to +150°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 100µA operation, the Burr-Brown REF200 Dual 100µA 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 ±15V supplies are available, the low cost precision OPA177 is recommended. For 5V single-supply applications, the OPA1013 Dual Single-Supply op amp is recommended. 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 measurement system. One of the 100µA 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 © 1991 Burr-Brown Corporation SBOA019 AB-036 VO OPA1013 R1 Motorola MTS102 VO = VBE (1 + R2/R1) – 100µA • R2 Where: VBE = voltage across diode (V) Zero and span adjustments with R1 and R2 are interactive. Figure 1. Simple Diode-based Temperature Measurement Circuit. interactive adjustment technique. Another possible disadvantage 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 VO = VBE (1 + R2/R1) – 100µA • R2 CALCULATING RESISTOR VALUES R1 = (δV /δT) • (V + TC • (TMIN –25°C)) – (TC • V1) O BE25 100µA • ((δVO/δT) – TC) R2 = R1 • ( (δVO/δT) TC – 1) Where: R1, R2 = Resistor values (Ω) VBE = Voltage across diode (V) VBE25 = Diode voltage at 25°C (V) Three choices are available for the MTS102—See table on page 2. Printed in U.S.A. September, 1991 V1 = Output voltage of circuit at TMIN (V) R2 = R1 • ( VO = Output voltage of circuit (V) TC = Diode temperature coefficient (V/°C) Others = as before δV O/δT = Desired output voltage change for given temperature change (V/°C) (Note: Must be negative for Figure 1 circuit.) EXAMPLE Design a temperature measurement system with a 0 to –1.0V output for a 0 to 100°C temperature. AVAILABLE VBE25 AND TC VALUES FOR MOTOROLA MTS102 TEMPERATURE SENSOR 0.580 0.595 0.620 –0.002315 –0.002265 –0.002183 – 1) RZERO = Zero (offset) adjust resistor (Ω) TMIN = Minimum process temperature (°C) TC (V/°C) TC Where: TC value depends on VBE25—See table below. VBE25 (V) (δVO/δT) TMIN = 0°C δVO/δT = (–1V – 0V)/(100°C – 0°C) = –0.01V/°C If VBE25 = 0.595V, TC = –0.002265V/°C, and RZERO = 1kΩ (use 2kΩ pot) R1 = 9.717kΩ EXAMPLE Design a temperature measurement system with a 0 to –1.0V output for a 0 to 100°C temperature. R2 = 33.18kΩ TMIN = 0°C For a 0 to –10V output with a 0 to 100°C temperature: RZERO = 1kΩ (use 2kΩ pot) δVO/δT = (–1V – 0V)/(100°C – 0°C) = –0.01V/°C R1 = 7.69kΩ R2 = 331.8kΩ If VBE25 = 0.595V, TC = –0.002265V/°C, and R1= 8.424kΩ R2= 28.77kΩ 4.5V to 36V For a 0 to –10V output with a 0 to 100°C temperature: R1= 6.667kΩ R2= 287.7kΩ REF200 100µA If independent adjustment of offset and span is required consider the circuit shown in Figure 2. In this circuit, a third resistor, RZERO is added in series with the temperaturesensing diode. System zero (offset) can be adjusted with RZERO without affecting span (gain). To trim the circuit adjust span first. Either R1 or R2 (or both) can be used to adjust span. As with the Figure 1 circuit this circuit has the possible disadvantage that the temperature to voltage conversion is inverting. 100µA R2 A1 OPA1013 Motorola MTS102 RZero The following relationships can be used to calculate nominal resistor values for the Figure 2 circuit. BASIC TRANSFER FUNCTION VO = (VBE + 100µA • RZERO) • (1 + R2/R1) – 100µA • R2 VO R1 VO = (VBE + 100µA • RZERO) • (1 + R2/R1) – 100µA • R2 Where: VBE = voltage across diode (V) Adjust span first with R1 or R2 then adjust zero with RZERO for noninteractive trim. CALCULATING RESISTOR VALUES Set RZERO = 1kΩ (or use a 2kΩ pot) R1 = Figure 2. Diode-based Temperature Measurement Circuit with Independent Span (gain) and Zero (offset) Adjustment. (δVO/δT) • (VBE25 + (RZERO • 100µA) + TC • (TMIN – 25°C)) – (TC • V1) 100µA • ((δVO/δT) – TC) 2 For a noninverting temperature to voltage conversion, consider the circuit shown in Figure 3. This circuit is basically the same as the Figure 2 circuit except that the amplifier is connected to the low side of the diode. With this connection, the temperature to voltage conversion is noninverting. As before, if adjustment is required, adjust span with R1 or R2 first, then adjust zero with RZERO. R1 RZero R2 A1 Motorola MTS102 A disadvantage of the Figure 3 circuit is that it requires a negative power supply. OPA1013 The following relationships can be used to calculate nominal resistor values for the Figure 3 circuit. VO REF200 100µA 100µA BASIC TRANSFER FUNCTION VO = (–VBE – 100µA • RZERO) • (1 + R2/R1) + 100µA • R2 –VS VO = (–VBE – 100µA • RZERO) • (1 + R2/R1) + 100µA • R2 Where: VBE = voltage across diode (V) CALCULATING RESISTOR VALUES R1 = same as Figure 2 R2 = same as Figure 2 Adjust span first with R1 or R2 then adjust zero with RZERO for noninteractive trim. Where: Components = as before Figure 3. Positive Transfer Function Temperature Measurement Circuit with Independent Span (gain) and Zero (offset) Adjustment. EXAMPLE Design a temperature measurement system with a 0 to 1.0V output for a 0 to 100°C temperature. BASIC TRANSFER FUNCTION VO = 100µA • RZERO • (1 + R2/R1) – VBE • R2/R1 TMIN = 0°C δVO/δT = (1V – 0V)/(100°C – 0°C) = 0.01V/°C CALCULATING RESISTOR VALUES If VBE25 = 0.595V, TC = –0.002265V/°C, and RZERO = RZERO = 1kΩ R1 = 9.717kΩ (TC • V1) – (δVO/δT) • (VBE25 + TC • (TMIN – 25°C)) 100µA • (TC – (δVO/δT)) R1 = 10kΩ (arbitrary) R2 = 33.18kΩ For a 0 to 10V output with a 0 to 100°C temperature: R2 = –R1 • ( RZERO = 1kΩ R1 = 7.69kΩ (δVO/δT) TC ) Where: Components = as before R2 = 331.8kΩ For a single-supply noninverting temperature to voltage conversion, consider the Figure 4 circuit. This circuit is similar to the Figure 2 circuit, except that the temperaturesensing diode is connected to the inverting input of the amplifier and the offsetting network is connected to the noninverting input. To prevent sensor loading, a second amplifier is connected as a buffer between the temp sensor and the amplifier. If adjustment is required, adjust span with R1 or R2 first, then adjust zero with RZERO. EXAMPLE Design a temperature measurement system with a 0 to 1.0V output for a 0 to 100°C temperature. TMIN = 0°C δVO/δT = (1V – 0V)/(100°C – 0°C) = 0.01V/°C If VBE25 = 0.595V, TC = –0.002265V/°C, and The following relationships can be used to calculate nominal resistor values for the Figure 4 circuit. RZERO = 5.313kΩ R1= 10.0kΩ R2= 44.15kΩ 3 For a 0 to 10V output with a 0 to 100°C temperature: The following relationships can be used to calculate nominal resistor values for the Figure 5 circuit. RZERO = 6.372kΩ R1 = 10.0kΩ BASIC TRANSFER FUNCTION R2 = 441.5kΩ VO = ((VBE2 + 100µA • RZERO2) – (VBE1 + 100µA • RZERO1)) • GAIN 4.5V to 36V Where: GAIN = 2 + 2 • R1/RSPAN REF200 CALCULATING RESISTOR VALUES 100µA 100µA RSPAN = –2 • R1 • TC (δVO/δT) + 2 • TC RZERO1 = RZERO2 = 500Ω (use 1kΩ pot for RZERO) A1 OPA1013 R1 R2 Where: A2 OPA1013 RSPAN = Span (gain) adjust resistor [Ω] VO Others = as before RZERO Motorola MTS102 EXAMPLE Design a temperature measurement system with a 0 to 1.0V output for a 0 to 1°C temperature differential. VO = 100µA • RZER O • (1 + R2/R1) – VBE • R2/R1 TMIN = 0°C Where: VBE = voltage across diode [V] δVO/δT = (1V – 0V)/(1°C – 0°C) = 1.0V/°C Adjust span first with R1 or R2 then adjust zero If VBE25 = 0.595V, TC = –0.002265V/°C, and with RZERO for noninteractive trim. RZERO = 1kΩ pot Figure 4. Single-supply Positive Transfer Function Temperature Measurement Circuit with Independent Span (gain) and Zero (offset) Adjustment. R1, R2, R3, R4 = 100kΩ, 1% RSPAN = 455Ω For differential temperature measurement, use the circuit shown in Figure 5. In this circuit, the differential output between two temperature sensing diodes is amplified by a two-op-amp instrumentation amplifier (IA). The IA is formed from the two op amps in a dual OPA1013 and resistors R1, R2, R3, R4, and RSPAN. RSPAN sets the gain of the IA. For good common-mode rejection, R1, R2, R3, and R4 must be matched. If 1% resistors are used, CMR will be greater than 70dB for gains over 50V/V. Span and zero can be adjusted in any order in this circuit. For a 0 to 10V output with a 0 to 1°C temperature differential: RZERO = 1kΩ pot R1, R2, R3, R4 = 100kΩ, 1% RSPAN = 45.3Ω 4 4.5V to 36V REF200 RSPAN 100µA 100µA R1 100kΩ R2 100kΩ R3 100kΩ A1 R4 100kΩ OPA1013 A2 OPA1013 Motorola MTS102 Motorola MTS102 RZERO VO VO = ((VBE2 + 100µA • RZERO2) – (VBE1 • + 100µA • RZERO1)) • GAIN GAIN = 2 + 2 • R1/RSPAN Adjust zero and span in any order. Figure 5. 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