® IXR100 Isolated, Self-Powered, Temperature Sensor Conditioning 4-20mA TWO-WIRE TRANSMITTER FEATURES APPLICATIONS ● 1500Vrms ISOLATION ● TRUE TWO-WIRE OPERATION : Power and Signal on One Wire Pair ● RESISTANCE OR VOLTAGE INPUT ● INDUSTRIAL PROCESS CONTROL: All Types of Isolated Transmitters; Pt100 RTD Thermocouple Inputs Current Shunt (mV) Inputs ● ISOLATED DUAL CURRENT SOURCES ● AUTOMATED MANUFACTURING ● DUAL MATCHED CURRENT SOURCES: 400µA at 7V ● WIDE SUPPLY RANGE 12V TO 36V ● PT100 RTD LINEARIZATION ● POWER PLANT/ENERGY MONITORING ● GROUND LOOP ELIMINATION DESCRIPTION The IXR100 is an isolated 2-wire transmitter featuring loop powered operation and resistive temperature sensor conditioning (excitation and linearization). It contains a DC/DC convertor, high accuracy instrumentation amplifier with single resistor programmable span and linearization, and dual matched excitation current sources. This combination is ideally suited to a range of transducers such as thermocouples, RTDs, thermistors and strain gages. The small size makes it ideal for use in head mounted isolated temperature transmitters as well as rack and rail mounted equipment. The isolated two-wire transmitter allows signal transmission and device power to be supplied on a single wire-pair by modulating the power supply current with the isolated signal source. The transmitter is resistant to voltage drops from long runs and noise from motors, relays, actuators, switches, transformers and industrial equipment. It can be used by OEMs producing isolated transmitter modules or by data acquisition system manufacturers. The IXR100 is also useful for general purpose isolated current transmission where the elimination of ground loops is important. 0.4mA Optional Offset Adjust 0.4mA RO Pt100 NONLINEARITY CORRECTION USING IXR100 4 2 +VIN 1 + +IR 4-20mA 10 IR 4.4 11 RO R OR Uncorrected Nonlinearity (%) 6 12 O VS 28 RS RS IXR100 + 7 RS RLIN 3 –VIN Corrected +0.1 – RLIN Com 5 18 RL – 9 8 RTD –0.1 RZ RLIN R CM 850 –200 0.01µF Process Temperature (°C) International Airport Industrial Park • Mailing Address: PO Box 11400 Tel: (520) 746-1111 • Twx: 910-952-1111 • Cable: BBRCORP • • Tucson, AZ 85734 • Street Address: 6730 S. Tucson Blvd. • Tucson, AZ 85706 Telex: 066-6491 • FAX: (520) 889-1510 • Immediate Product Info: (800) 548-6132 © PDS-1141A 1992 Burr-Brown Corporation Printed in U.S.A. August, 1993 VOUT SPECIFICATIONS ELECTRICAL At VS = +24V, TA = +25°C, unless otherwise noted. IXR100 PARAMETER OUTPUT AND LOAD CHARACTERISTICS Output Current Output Current Limit Loop Supply Voltage Load Resistance CONDITIONS MIN Linear Operating Region 4 TYP UNITS 20 mA mA VDC Ω 32 11.6 RLOAD = (VS –11.6)/IO ZERO Initial Error (1) vs Temperature 36 VIN = 0, RS = ∞ SPAN Output Current Equation Span Equation Untrimmed Error vs Temperature Nonlinearity : EMF Input : Pt100 Input INPUT Voltage Range Common-Mode Range Offset Voltage vs Temperature vs Supply MAX RS in Ω, VIN in V 300 200 µA ppm FSR/°C 0 100 0.025 A/V % ppm/°C %FSR %FSR 4 2.5 5 V V mV µV/°C dB IO = 4mA + [0.016 + (40/RS)] (VIN) S = [0.016 + (40/RS)] (1) –2.5 Excluding TCR of RS 50 0.01 0.1 (2) (3) RS = ∞ VIN+, VIN– with Respect to COM 1 2 0.5 3 100 CURRENT SOURCES Magnitude Accuracy vs Temperature Match vs Temperature 0.4 50 25 DYNAMIC RESPONSE Settling Time To 0.1% of Span 500 TEMPERATURE RANGE Operating Storage ISOLATION Isolation Voltage 1 100 0.5 50 –20 –40 VISO VISO 1000 JP 1500 KP mA % ppm/°C % ppm/°C ms +70 +85 °C °C Vrms Vrms NOTES: (1) Can be adjusted to zero. (2) End point span non-linearity. (3) End point, corrected span non-linearity with a Pt100 RTD input operated from –200°C to +850°C. ELECTROSTATIC DISCHARGE SENSITIVITY ABSOLUTE MAXIMUM RATINGS Power Supply (+VS –IOUT) .................................................................. 40V Input Voltage (Com to VIN) .................................................................... 9V Storage Temperature Range ........................................... –40°C to +85°C Lead Temperature (soldering 10s) ................................................ +300°C Output Current Limit Duration ................................................. Continuous Power Dissipation ......................................................................... 500mW Electrostatic discharge can cause damage ranging from performance degradation to complete device failure. BurrBrown Corporation recommends that this integrated circuit be handled and stored using appropriate ESD protection methods. PACKAGE INFORMATION MODEL PACKAGE PACKAGE DRAWING NUMBER(1) IXR100 2-wire Transmitter 901 NOTE: (1) For detailed drawing and dimension table, please see end of data sheet, or Appendix D of Burr-Brown IC Data Book. The information provided herein is believed to be reliable; however, BURR-BROWN assumes no responsibility for inaccuracies or omissions. BURR-BROWN assumes no responsibility for the use of this information, and all use of such information shall be entirely at the user’s own risk. Prices and specifications are subject to change without notice. No patent rights or licenses to any of the circuits described herein are implied or granted to any third party. BURR-BROWN does not authorize or warrant any BURR-BROWN product for use in life support devices and/or systems. ® IXR100 2 PIN CONFIGURATION Top View IREF1 1 28 +VS VIN+ 2 27 * VIN– 3 26 * IREF2 4 25 * Com 5 24 * RS1 6 23 * RS2 7 22 * RL1 8 21 * RL2 9 20 * OS1 10 19 * OS2 11 18 IOUT OS3 12 17 * * 13 16 * * 14 15 * * = No Pin DISCUSSION OF PERFORMANCE FUNCTIONAL DESCRIPTION The IXR100 comprises of several functions: • Sensor excitation The IXR100 makes the design of isolated two wire 4 to 20mA transmitters easy and provides exceptional performance at very low cost. It combines several unique features not previously available in a single package. These include galvanic isolation, sensor excitation and linearization, excellent DC performance, and low zero and span drift. The IXR100 functions with voltages as low as 11.6V at the device. This allows operation with power supplies at or below 15V. When used with the RCV420 the complete 4 to 20mA current loop requires only 13.1V. If series diode protection is desired the minimum loop supply voltage is still only about 13.7V. This is especially useful in systems where the available supplies are only 15V. • Internal voltage regulator • Input amplifier and V/I converter • Linearization circuit • DC/DC Converter SENSOR EXCITATION Sensor Excitation consists of two matched 0.4mA current sources. One is used to excite the resistive sensor and the other is used to excite the zero balance resistor RZ. When the linearity correction feature is used these current sources are modulated together so that three wire operation of a Pt100 RTD is possible. BASIC CONNECTION The basic connection of the IXR100 is shown in Figure 1. A differential voltage applied between pins 2 and 3 will cause a current of 4 to 20mA to circulate in the two wire output loop pins 28 and 18. Pins 1 and 4 supply the current excitation for resistive sensors. Pins 6 and 7 are provided for the connection of an external span resistor which increases the gain. Pins 8 and 9 provide linearity correction. Pins 10, 11 and 12 adjust the output offset current. INTERNAL VOLTAGE REGULATOR The circuitry within the IXR100 regulates the supply voltage to the DC/DC Converter, Input Amplifier, Linearization Amplifier and V/I Converter and removes the normal variations in VS from these stages as the output spans from 4 to 20mA. ® 3 IXR100 0.4mA IO = 4mA + (0.016 + 40 ) VIN RS Optional Offset Adjust 0.4mA RO VIN = IREF (RTD – RZ) 4 2 +VIN 1 + +IR 4-20mA 10 IR 11 RO R 12 OR O 6 VS 28 RS (2) RS IXR100 + 7 RS RLIN 3 –VIN RL – 18 Com 5 VOUT 9 – RLIN 8 RTD RZ (1) RLIN (3) R CM NOTES: (1) RZ = RTD resistance at the minimum process temperature. 40 (2) RS = Ω. 0.016/(∆VIN) – 0.016 (3) RLIN = 500Ω to 1500Ω or ∞ if linearization is not required. 0.01µF FIGURE 1. Basic Connection for RTD. INPUT AMPLIFIER AND V/I CONVERTER DC/DC CONVERTER The Input Amplifier is an instrumentation amplifier whose gain is set by RS, it drives the V/I Converter to produce a 4 to 20mA output current. The Input Amplifier has a common mode voltage range of 2 to 4V with respect to COM (pin 5). Normally this requirement is satisfied by returning the currents from the RTD and zero balance resistor RZ to COM through a common mode resistor RCM. For most applications a single value of 3.9kΩ may be used. When used with RTDs having large values of resistance RCM must be chosen so that the inputs of the amplifier remain within its rated common mode range. RCM should be bypassed with a 0.01µF or larger capacitor. The DC/DC Converter transfers power from the 2 wire current loop across the barrier to the circuitry used on the input side of the isolation barrier. PIN DESCRIPTIONS IREF1, IREF2 These pins provide a matched pair of current sources for sensor excitation. These current sources provide excellent thermal tracking, and when the linearization feature is used, are modulated by an equal amount. Their nominal current value is 0.4mA and their compliance voltage is: VIN+ < VIREF < (Com + 7V) LINEARIZATION CIRCUIT The Linearity Correction Circuit is unique in several ways. A single external resistor will provide up to 50 times improvement in the basic RTD linearity. Terminal based non-linearity can be reduced to less than ±0.1% for all RTD temperature spans. The Linearization circuit also contains an instrumentation amplifier internally connected to the ±VIN pins. The gain of this stage is set by RLIN. The output controls the excitation current sources to produce an increasing excitation current as VIN increases. An important feature is that the Linearity Correction is made directly to the RTD output independent of the gain of the Input Amplifier. This provides minimal interaction between RS and RZ. This feature can be useful at the systems level by reducing data acquisition system processor overhead previously used to linearize sensor response in software/firmware. IREF = 400µA + +VIN, –VIN These are the inputs to both the input amplifier and the linearization amplifier. Because the IXR100 has been optimized for RTD applications, the two sets of inputs are internally connected. RS1, RS2 The resistor connected across these terminals determines the gain of the IXR100. For normal 4-20mA outputs: 40 Ω (1) R = S 0.016/(∆VIN) – 0.016 ® IXR100 VREF 2RLIN 4 RL1, RL2 The resistor connected between these terminals determines the gain of the linearization circuit and the amount of correction applied to the RTD. Its value may be determined in several ways. Two of which are shown as follows. – 1. Empirically by interactively adjusting RLIN, RS and RZ to achieve best fit 4 to 20mA output. RZ is used to set 4mA at minimum input, RS is adjusted for 12mA with a half span input, and RLIN is adjusted to give 20mA with a full span input. This may require a few iterations but is probably the most practical method for field calibration. RLIN will range between 500Ω and 1500Ω for 100Ω sensors (Pt100, D100, SAMA). Initially it may seem a little strange adjusting RS for 12mA and RLIN for 20mA. However, convergence is achieved much more quickly as the linearized curve passes through zero and has less effect at the mid span and the linearity trim resistor tends to adjust the transfer function more at the full span than the mid point. IXR100 11 + (a) 12 10 10kΩ ±400µA adjust range – 2. Using Table I and linear interpolation for values of span not given in the table. This will yield very accurate results for the Pt100 sensor and acceptable results for D100 and SAMA sensors. IXR100 ZERO ADJUST (OPTIONAL) OS1, OS2, OS3 The IXR100 has provision for adjusting the output offset current as shown in Figure 2. In many applications the already low offset will not need to be known at all. This trim effects the V/I converter stage and does not introduce VOS drift errors that occur when the trim is performed at the input stage. If possible use RZ to trim sensor output error to zero and use the offset control to trim the output to 4mA when VIN = 0V. The offset adjustment can be made with a 11 + (b) 12 10 5kΩ 5kΩ ±40µA adjust range FIGURE 2. Basic Connection for Zero Adjust. SPAN ∆T (°C) TMIN (°C) 50 100 200 300 400 500 600 700 800 900 1000 –200 –150 –100 –50 573 745 983 1233 653 855 1105 1284 839 1059 1228 1286 995 1158 1251 1262 1083 1197 1249 1236 1131 1206 1231 1208 1152 1205 1207 1180 1159 1196 1182 1152 1159 1175 1156 1125 1154 1151 1129 1097 1140 1127 0 50 100 150 1302 1263 1225 1188 1287 1249 1211 1174 1273 1220 1183 1146 1229 1192 1155 1119 1201 1164 1127 1091 1173 1136 1100 1064 1145 1108 1073 1038 1117 1081 1046 1011 1089 1054 200 250 300 350 1151 1114 1079 1044 1137 1101 1066 1031 1110 1074 1039 1005 1083 1048 1013 979 1056 1021 987 954 1030 995 962 928 1003 969 400 450 500 550 1009 975 942 909 996 963 930 897 971 938 905 873 946 913 881 849 921 888 600 650 700 750 800 877 845 814 784 754 865 834 803 773 841 810 NOTES: (1) Linear interpolation between two horizontal or vertical values yields acceptable values. (2) Although not optimum, these values will also yield acceptable results with D100 and SAMA 100Ω nominal sensors. (3) Double RLIN value for PT200. TABLE I. RLIN Values for Pt100 Sensor. ® 5 IXR100 the effects of non-uniform fields existing in heterogeneous dielectric material during barrier degradation. In the case of void non-uniformities, electric field stress begins to ionize the void region before bridging the entire high voltage barrier. potentiometer connected as shown in Figures 2a and 2b. The circuit shown in Figure 2a provides more range while the circuit in Figure 2b provides better resolution. Note, it is not recommended to use this adjusting procedure for zero elevation or suppression. See the signal suppression and elevation section for the proper techniques. The transient conduction of charge during and after the ionization can be detected externally as a burst of 0.01µs-0.1µs current pulses that repeat on each AC voltage cycle. The minimum AC barrier voltage that initiates partial discharge is defined as the “inception voltage”. Decreasing the barrier voltage to a lower level is required before partial discharge ceases and is defined as the “extinction voltage”. COM This is the return for the two excitation currents IREF1 and IREF2 and is the reference point for the inputs. VS, IOUT These are the connections for the current loop VS being the most positive connection. For correct operation these pins should have 11.6 to 36V between them. We have designed and characterized the package to yield an inception voltage in excess of 2400Vrms so that transient overvoltages below this level will not cause any damage. The extinction voltage is above 1500Vrms so that even overvoltage-induced partial discharge will cease once the barrier voltage is reduced to the rated level. Older high voltage test methods relied on applying a large enough overvoltage (above rating) to catastrophically break down marginal parts, but not so high as to damage good ones. Our new partial discharge testing gives us more confidence in barrier reliability than breakdown/no breakdown criteria. HIGH VOLTAGE TESTING Burr-Brown Corporation has adopted a partial discharge test criterion that conforms to the German VDE0884 Optocoupler Standards. This method requires the measurement of minute current pulses (< 5pC) while applying 2400rms, 60Hz highvoltage stress across every devices isolation barrier. No partial discharge may be initiated to pass this test. This criterion confirms transient overvoltage (1.6 x VRATED) protection without damage. Life-test results verify the absence of failure under continuous rated voltage and maximum temperature. APPLYING THE IXR100 The IXR100 has been designed primarily to correct nonlinearities inherent in RTD sensors. It may also be used in other applications where its excellent performance makes it superior to other devices available. Examples are shown in the Applications Section. This new test method represents the “state-of-the-art” for nondestructive high voltage reliability testing. It is based on Optional Input Filtering 0.4mA 0.4mA 1 (1) R1 3 4 – 28 1N4148 7 C1 TRANZORB VS RS IXR100 CBYPASS + RL – 6 18 (1) R2 2 CBYPASS + 9 5 8 R LIN CBYPASS 0.01µF RZ RTD = Transmitter Case 3.9kΩ NOTE: (1) R1 and R 2 should be made equal if used (±1% resistors are adequate). FIGURE 3. Transient and RFI Protection Circuit. ® IXR100 6 VOUT RFI AND TRANSIENT SUPPRESSION Radio frequency interference and transients are a common occurrence in 4-20mA loops, especially when long wiring lengths are involved. RFI usually appears as a temporary change in output and results from rectification of the radio signal by one or more stages in the amplifier. For sensors which are closely coupled to the IXR100 and are contained in a common metal housing, the usual entry for RFI is via the 4-20mA loop wiring. Coaxial bypass capacitors may be used with great effectiveness to bring these leads into the transducer housing while suppressing the RFI. Values of 100 to 1000pF are generally recommended. For sensors remote from the IXR100, coaxial capacitors can also be used to filter the excitation and signal leads. Additional low-pass filtering at the IXR100 input helps suppress RFI. The easiest way to do this is with the optional differential RC filter shown in Figure 4. Typical values for R1 and R2 are 100-1000Ω, and for C1 are 100-1000pF. INPUT BANDWIDTH LIMITING Filtering at the input to the IXR100 is recommended where possible and can be done as shown in Figure 4. C1 connected to pins 3 and 4 will reduce the bandwidth with a f–3dB frequency given by: f–3dB = 0.159/(R1 + R2 + RTD + RZ) (C1 + 3pF) This method has the disadvantage of having f–3dB vary with R1, R2, RTD, and RZ may require large values of R1, and R2. R1 and R2 should be matched to prevent zero errors due to input bias current. SIGNAL SUPPRESSION AND ELEVATION In some applications it is desired to have suppressed zero range (span elevation) or elevated zero range (span suppression). This is easily accomplished with the IXR100 by using the current sources to create the suppression/elevation voltage. The basic concept is shown in Figure 5. In this example the sensor voltage is derived from RT (a thermistor, RTD or other variable resistance element) excited by one of the 0.4mA current sources. The other current source is used to create the elevated zero range voltage. Figures 6a, 6b, 6c and 6d show some of the possible circuit variations. These circuits have the desirable feature of noninteractive span and suppression/elevation adjustments. Transient suppression for negative voltages can be provided by the reverse-polarity protection diodes discussed later. However, positive transients cannot be handled by these diodes and do frequently occur in field-mounted loops. A shunt zener diode is of some help, but most zener diodes suffer from limited current-handling capacity and slow turnon. Both of these characteristics can lead to device failure before the zener conducts. One type of zener, called the TRANZORB and available from General Semiconductor Industries, is especially effective in protecting against highenergy transients such as those induced by lightning or motor contactors. Choose a TRANZORB with a voltage rating close to, but exceeding, the maximum VS which the IXR100 will see. In combination, the coaxial bypass capacitors and TRANZORB provide a very high level of protection against transients and RFI. 0.4mA 0.4mA NOTE: Use of the optional offset null (pins 10, 11, and 12) for elevation or suppression is not recommended. This trim technique is used only to trim the IXR100’s output offset current. MAJOR POINTS TO CONSIDER WHEN USING THE IXR100 1. The leads to RS and RLIN should be kept as short as possible to reduce noise pick-up and parasitic resistance. If the linearity correction feature is not desired, the RLIN pins are left open. 2. +VS should be bypassed with a 0.01µF capacitor as close to the unit as possible (pins 18 to 28). 1 (1) R1 3 4 3. Always keep the input voltages within their range of linear operation, +2V to +4V (±VIN measured with respect to pin 5). – 7 C1 RS 20 IXR100 Span Adjust 6 15 2 9 + i 0 (mA) (1) R2 5 10 8 R LIN 5 0.01µF RZ Elevated Zero Range Suppressed Zero Range RTD 3.9kΩ 0 – 0 + NOTE: (1) R 1 and R 2 should be made equal if used. VIN Figure 4. Optional Bandwidth-Limiting Circuitry. Figure 5. Elevation and Suppression Graph. ® 7 IXR100 0.4mA 0.4mA 0.4 mA – VIN + + e'2 + V4 + R4 e'2 – 0.4mA – RT RT + V4 – – VIN + R4 – 0.8mA 0.8mA VIN = (e'2 –V4) V4 = 0.4mA X R4 e'2 = 0.4mA X RT VIN = (e'2 +V4) V4 = 0.4mA X R4 e'2 = 0.4mA X RT (a) Elevated Zero Range (b) Suppressed Zero Range 0.8mA 0.8mA – VIN + – e'2 – VIN + + + V4 R4 – + + e'2 – V4 R4 – 0.8mA 0.8mA VIN = (e'2 –V4) VIN = (e'2 +V4) V4 = 0.8mA X R4 V4 = 0.8mA X R4 (c) Elevated Zero Range (d) Suppressed Zero Range FIGURE 6. Elevation and Suppression Circuits. 4. The maximum input signal level (∆VIN) is 1V with RS open and is less as RS decreases in value. important if the receiving equipment has particularly low resistance or uses higher voltage supplies. In general, the series diode is recommended unless 12V operation is necessary. In either case a 1N4148 diode is suitable. 5. Always return the current references to COM (pin 5) through an appropriate value of RCM to keep VCM within its operating range. Also, operate the current sources within their rated compliance voltage: VIN + ≤ VIREF ≤ (Com + 7V) 8. Use a layout which minimizes parasitic inductance and capacitance, especially in high gain. RECOMMENDED HANDLING PROCEDURES FOR INTEGRATED CIRCUITS All semiconductor devices are vulnerable, in varying degrees, to damage from the discharge of electrostatic energy. Such damage can cause performance degradation or failure, either immediate or latent. As a general practice, we recommend the following handling procedures to reduce the risk of electrostatic damage. 6. Always choose RL, (including line resistance) so that the voltage between pins 18 and 28 (+VS) remains within the 11.6V to 36V range as the output changes between 4mA and 20mA. 7. It is recommended that a reverse polarity protection diode be used. This will prevent damage to the IXR100 caused by a transient or long-term reverse bias between pins 18 and 28. This diode can be connected in either of the two positions shown in Figure 7, but each connection has its trade-off. The series-connected diode will add to the minimum voltage at which the IXR100 will operate but offers loop and device protection against both reverse connections and transients. The reverse-biased diode in parallel with the IXR100 preserves 11.6V minimum operation and offers device protection, but could allow excessive current flow in the receiving instrument if the field leads are accidently reversed. This is particularly 1. Remove static-generating materials, such as untreated plastic, from all areas where microcircuits are handled. 2. Ground all operators, equipment, and work stations. 3. Transport and ship microcircuits, or products incorporating microcircuits, in static-free, shielded containers. 4. Connect together all leads of each device by means of a conductive material, when the device is not connected into a circuit. ® IXR100 8 5. Control relative humidity to as high a value as practical (50% recommended). equally, so that use of the linearity correction does not affect the cancellation. This action is true so long as the three wires are of the same length and gauge. Because most RTD leads are twisted and bundled, this requirement is usually met with no difficulty. Care must be taken that intermediate connections such as screw terminals do not violate this assumption by introducing unequal line resistances. RTD APPLICATIONS The IXR100 has been designed with RTD applications specifically in mind. The following information provides additional information for those applications. RTD ZERO ELEVATION AND SUPPRESSION The IXR100 may be operated in zero-elevated and zerosuppressed ranges by simply offsetting RZ. It may also be used in increase-decrease applications by interchanging the physical locations of the RTD and RZ as shown in Figure 8. Use the same values of RZ, RLIN and RS. Again, because the current sources are matched and are modulated equally, this connection has no effect on IXR100 performance, especially in three-wire applications. TWO- AND THREE-WIRE CONNECTIONS The IXR100 performs well with two-wire and three-wire RTD connections commonly encountered in industrial monitoring and control. In two-wire applications, the voltage drop between the RTD and the IXR100 can be nulled by proper adjustment of RZ, but care must be taken that this voltage drop does not vary with ambient conditions. Such variation will appear as an apparent variation in the RTD resistance and therefore as a change in measured temperature. Also, the linearity correction will interpret this change as a variation and attempt to linearize both the actual RTD signal and the resistance changes in the signal lines. For these reasons, the line resistance between the RTD and the IXR100 should be minimized by keeping line lengths short and/or using largegauge wires. This limitation does not apply for three-wire connections. OPEN CIRCUIT DETECTION In some applications of the IXR100, the RTD will be located remotely. In these cases, it is possible for open circuits to develop. The IXR100 responds in the following manner to breaks in each lead. The following connections refer to the RTD connections shown in Figure 7. In three-wire applications, shown in Figure 7, the RTD and RZ lead arrangements set up a pseudo-Kelvin connection to the RTD. This occurs because the currents through the three wires are set up in opposing directions and cancel IR drops in the RTD leads. The current sources are both modulated TERMINAL OPEN IOUT(1) 1 2 3 32mA 3.6mA 32mA NOTE: (1) Approximate value. 1 3 4 – D1 +VS 28 1N4148 7 4-20mA – 0.4mA 0.4mA VIN RS IXR100 VOUT + 6 + 18 2 + 9 RL – 5 8 1 RLIN RZ 2 RTD 3 R CM = 3.9kΩ Three-wire Connection 0.01µF FIGURE 7. Basic 3-Wire RTD Connection for Increase-Increase Action. ® 9 IXR100 1 3 4 – D1 +VS 28 1N4148 7 4-20mA – 0.4mA 0.4mA VIN RS IXR100 VOUT + 6 + 18 2 9 + RL – 5 8 1 RLIN RZ RTD 2 3 RCM = 3.9kΩ Three-wire Connection 0.01µF FIGURE 8. Basic 3-Wire RTD Connection for Increase-Decrease Action. OTHER APPLICATIONS From Equation (1), RS = 48.5Ω. Span adjustment (calibration) is accomplished by trimming RS. In instances where the linearization capability of the IXR100 is not required, it can still provide improved performance in several applications. Its small size, wide compliance voltage, low zero and span drift, high PSRR, high CMRR and excellent linearity makes the IXR100 ideal for a variety of other isolated two-wire transmitter applications. It can be used by OEMs producing different types of isolated transducer transmitter modules and by data acquisition systems manufacturers who gather transducer data. Current mode transmission greatly reduces noise interference. The twowire nature of the device allows economical signal conditioning at the transducer. Thus, the IXR100 is, in general, very suitable for a wide variety of applications. Some examples, including an isolated non-linearized Pt100 case, follow. In order to make the lower range limit of 25°C correspond to the output lower range limit at 4mA, the input circuitry shown in Figure 9 is used. VIN must be 0V at 25°C and RZ is chosen to be equal to the RTD resistance at 25°C, or 109.73Ω. Computing RCM and checking CMV: At +25°C, VIN+ = 43.9mV At +150°C, VIN+ = 62.9mV Since both VIN+ and VZ are small relative to the desired 2V common-mode voltage, they may be ignored in computing RCM as long as the CMV is met. RCM = 3V/0.8mA = 3.75kΩ VIN+ min = 3V + 0.0439V VIN+ max = 3V + 0.0629V EXAMPLE 1 VIN– = 3V + 0.0439V Pt100 RTD without linearization shown in Figure 9. EXAMPLE 2 Given a process with temperature limits of +25°C and +150°C, configure the IXR100 to measure the temperature with a Pt100 RTD which produces 109.73Ω at 25°C and 157.31Ω at 150°C (obtained from standard RTD tables). Transmit 4mA for +25°C and 20mA for +150°C. The change in resistance of the RTD is 47.6Ω. When excited with a 0.4mA current source ∆VIN is 0.4mA x 47.6Ω = 19mV. Thermocouple shown in Figure 10. RS = 40 Ω 0.016/(∆VIN) – 0.016 Given a process with temperature (Tl) limits of 0°C and +1000°C, configure the IXR100 to measure the temperature with a Type J thermocouple that produces a 58mV change for 1000°C change. Use a semiconductor diode for a cold junction compensation to make the measurement relative to 0°C. This is accomplished by supplying a compensating voltage, equal to that normally produced by the thermocouple with its “cold junction” (T2) at ambient. At +25°C this is 1.28mV (from thermocouple tables with reference junction at 0°C). Typically, at T2 = +25°C, VD = 0.6V and (1) ® IXR100 10 ∆VD/∆T = –2mV/°C. R5 and R6 form a voltage divider for the diode voltage VD. The divider values are selected so that the gradient ∆VD/∆T equals the gradient of the thermocouple at the reference temperature. At +25°C this is approximately –52µV/°C (obtained from standard thermocouple table); therefore, ∆VTC /∆T = (∆VD /∆T)(R6 /(R5 + R6 )) VD(25°C) = 600mV VIN(25°C) = 600mV (100/3740) = 16.0mV VIN = VIN+ – VIN– = VTC + V4 – VIN– With VIN = 0 and VTC = –1.28mV, V4 = VIN+ – VTC (2) V4 = 16.0mV – (–1.28mV) –52µV/°C = (–2000µV/°C)(R6 /(R5+R6 )) 0.4mA (R4) = 17.28mV R5 is chosen as 3.74kΩ to be much larger than the resistance of the diode. Solving for R6 yields 100Ω. R4 = 43.2Ω Transmit 4mA for Tl = 0°C and 20mA for Tl = +1000°C. Note: VlN = VIN+ – VIN– indicates that Tl is relative to T2. The input full scale span is 58mV. RS is found from Equation (1) and equals 153.9Ω. THERMOCOUPLE BURN-OUT INDICATION In process control applications it is desirable to detect when a thermocouple has burned out. This is typically done by forcing the two-wire transmitter current to the upper or lower limit when the thermocouple impedance goes very high. The circuits of Figures 10, 11 and 12 inherently have down scale indication. When the impedance of the thermocouple gets very large (open) the bias current flowing into the + input (large impedance) will cause IO to go to its lower range limit value (about 3.6mA). If up scale indication is desired, the circuit of Figure 13 should be used. When the TC opens, the output will go to its upper range limit value (about 32mA or higher). R4 is chosen to make the output 4mA at TTC = 0°C (VTC = 1.28mV) and TD = 25°C (VD = 0.6V). VTC will be –1.28mV when TTC = 0°C and the reference junction is at +25°C. V4 must be computed for TD = +25°C to make VIN = 0V. 1 3 0.4mA 4 – D1 0.4mA +VS 28 1N4148 7 4-20mA – VIN RS IXR100 VOUT + 6 + 18 2 RZ + RL – 5 RTD RCM 0.01µF FIGURE 9. Pt100 RTD Without Linearization. ® 11 IXR100 0.4mA 0.4mA R5 3.74kΩ 1 3 + VD – 1N4148 4 – +VS 28 7 + VIN– – RS R6 100Ω IXR100 153.9Ω 6 18 Thermocouple TTC 2 VTC – IOUT 0.01µF VIN+ + V4 – 5 + + + – R4 43.2Ω 3.9kΩ Temperature T1 Temperature T2 = TD FIGURE 10. Thermocouple Input Circuit with Two Temperature Regions and Diode (D) Cold Junction Compensation. 0.4mA 0.4mA 3.9kΩ This circuit has down scale burn-out indication. 1 3 4 – D1 Type J + – +VS 28 1N4148 7 4-20mA Zero Adjust 50Ω 100Ω RS IXR100 VOUT 6 + 18 2 RL – 5 + 3.9kΩ 0.01µF FIGURE 11. Thermocouple Input with Diode Cold Junction Compensation and Down Scale Burn-out Indication. ® IXR100 12 0.4mA 0.4mA This circuit has down scale burn-out indication. 1 3 4 – D1 Type J 51Ω 7 + – RTD 100Ω 50Ω +VS 28 1N4148 4-20mA RS IXR100 VOUT Zero Adjust 6 + RL – 18 2 5 + 3.9kΩ 0.01µF FIGURE 12. Thermocouple Input with RTD Cold Junction Compensation and Down Scale Burn-out Indication. This circuit has up scale burn-out indication. Type J 0.4mA 0.4mA 1 + – 3 4 – D1 7 51Ω 50Ω 4-20mA RS RTD 100Ω +VS 28 1N4148 IXR100 VOUT Zero Adjust 6 + RL – 18 2 Zero 5 + 3.9kΩ 0.01µF FIGURE 13. Thermocouple Input with RTD Cold Junction Compensation and Up Scale Burn-out Indication. RLIN 14V to 38V 1 4 8 2 0.4mA 0.4mA 7 IXR100 16 3 10 3 18 – 1µF + 6 RZ +VS 28 RS RTD 1N4148 9 + 11 12 15 5 RCV420 2 3.9kΩ 4 0.01µF 14 13 VOUT 0 - 5V 5 1µF + –VS FIGURE 14. Isolated 4-20mA Instrument Loop. ® 13 IXR100 +VS 1 –VS 4 REF200 100µA IR 2 VIN OPA177 R3 19.5kΩ IXR100 I IN 4-20mA –VS 0.01µF 400Ω R1 R2 3 100Ω COM IO R (1) = I IN( I + 2 ) + I R ( R 1 + R 2 + R 3) / R 1 = 1.25IIN – 5mA R1 IO 0-20mA NOTE: (1) Other conversions are readily achievable by changing the R1, R2, and R3 ratios (see Burr-Brown Application Bulletin AB-031). FIGURE 15. 4-20mA to 0-20mA Output Converter. ® IXR100 14