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features l 1500vrms isolation l true two-wire operation : power and signal on one wire pair l resistance or voltage input l dual matched current sources: 400 m a at 7v l wide supply range 12v to 36v l pt100 rtd linearization applications l industrial process control: all types of isolated transmitters; pt100 rtd thermocouple inputs current shunt (mv) inputs l isolated dual current sources l automated manufacturing l power plant/energy monitoring l ground loop elimination isolated, self-powered, temperature sensor conditioning 4-20ma two-wire transmitter 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 instru- mentation 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 tem- perature transmitters as well as rack and rail mounted equipment. +0.1 ?00 pt100 nonlinearity correction using IXR100 process temperature (?) nonlinearity (%) 850 4.4 corrected uncorrected ?.1 r z r s +v in + IXR100 r cm 0.01? rtd r lin r l + 0.4ma 0.4ma r o 4-20ma optional offset adjust ? in 8 9 3 7 6 2 4 1 11 10 12 28 18 5 v out v s +i r i r r o r o r o com r lin r lin r s r s IXR100 international airport industrial park ? mailing address: po box 11400 ? tucson, az 85734 ? street address: 6730 s. tucson blvd. ? tucson, az 85706 tel: (520) 746-1111 ? twx: 910-952-1111 ? cable: bbrcorp ? telex: 066-6491 ? fax: (520) 889-1510 ? immediate product info: (800) 548-6132 the isolated two-wire transmitter allows signal trans- mission 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. ? 1992 burr-brown corporation pds-1141a printed in u.s.a. august, 1993
IXR100 2 electrostatic discharge sensitivity electrostatic discharge can cause damage ranging from performance degradation to complete device failure. burr- brown corporation recommends that this integrated circuit be handled and stored using appropriate esd protection methods. specifications electrical at v s = +24v, t a = +25 c, unless otherwise noted. IXR100 parameter conditions min typ max units output and load characteristics output current linear operating region 4 20 ma output current limit 32 ma loop supply voltage 11.6 36 vdc load resistance r load = (v s C11.6)/i o w zero initial error (1) v in = 0, r s = 300 m a vs temperature 200 ppm fsr/ c span output current equation r s in w , v in in v i o = 4ma + [0.016 + (40/r s )] (v in ) span equation s = [0.016 + (40/r s )] a/v untrimmed error (1) C2.5 0 % vs temperature excluding tcr of r s 50 100 ppm/ c nonlinearity : emf input (2) 0.01 0.025 %fsr : pt100 input (3) 0.1 %fsr input voltage range r s = 1v common-mode range v in+ , v inC with respect to com 2 4 v offset voltage 0.5 2.5 mv vs temperature 35 m v/ c vs supply 100 db current sources magnitude 0.4 ma accuracy 1% vs temperature 50 100 ppm/ c match 0.5 % vs temperature 25 50 ppm/ c dynamic response settling time to 0.1% of span 500 ms temperature range operating C20 +70 c storage C40 +85 c isolation v iso 1000 jp vrms isolation voltage v iso 1500 kp 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 C200 c to +850 c. 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 users 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. absolute maximum ratings power supply (+v s Ci out ) .................................................................. 40v input voltage (com to v in ) .................................................................... 9v storage temperature range ........................................... C40 c to +85 c lead temperature (soldering 10s) ................................................ +300 c output current limit duration ................................................. continuous power dissipation ......................................................................... 500mw package information package drawing model package 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.
IXR100 3 pin configuration discussion of performance the IXR100 makes the design of isolated two wire 4 to 20ma transmitters easy and provides exceptional perfor- mance at very low cost. it combines several unique features not previously available in a single package. these include galvanic isolation, sensor excitation and linearization, excel- lent 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. 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. functional description the IXR100 comprises of several functions: ? sensor excitation ? 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 r z . when the linearity correction feature is used these current sources are modulated together so that three wire operation of a pt100 rtd is possible. 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 varia- tions in v s from these stages as the output spans from 4 to 20ma. i ref1 v in+ v in? i ref2 com rs 1 rs 2 rl 1 rl 2 os 1 os 2 os 3 * * +v s * * * * * * * * * i out * * * top view 1 2 3 4 5 6 7 8 9 10 11 12 13 14 28 27 26 25 24 23 22 21 20 19 18 17 16 15 * = no pin
IXR100 4 dc/dc converter 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 i ref1 , i ref2 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: v in + < v iref < (com + 7v) +v in , Cv in 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. r s1 , r s2 the resistor connected across these terminals determines the gain of the IXR100. for normal 4-20ma outputs: 0.016/( d v in ) C 0.016 40 r s = w input amplifier and v/i converter the input amplifier is an instrumentation amplifier whose gain is set by r s , 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 r z to com through a common mode resistor r cm . for most applica- tions a single value of 3.9k w may be used. when used with rtds having large values of resistance r cm must be chosen so that the inputs of the amplifier remain within its rated common mode range. r cm should be bypassed with a 0.01 m f or larger capacitor. 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 v in pins. the gain of this stage is set by r lin . the output controls the excitation current sources to produce an increas- ing excitation current as v in 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 r s and r z . 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. figure 1. basic connection for rtd. r z r s +v in + IXR100 r cm 0.01? i = 4ma + (0.016 + ) v v in = i ref (rtd ?r z ) o 40 r s in rtd r lin r l + (2) notes: (1) r z = rtd resistance at the minimum process temperature. (2) r s = w. 0.016/( d v in ) ?0.016 40 (3) r lin = 500 w to 1500 w or if linearization is not required. 0.4ma 0.4ma r o 4-20ma optional offset adjust (1) (3) ? in 8 9 3 7 6 2 4 1 11 10 12 28 18 5 v out v s +i r i r r o r o r o com r lin r lin r s r s (1) v ref 2r lin i ref = 400 m a +
IXR100 5 12 11 + IXR100 10 10k w + IXR100 5k w 5k w (a) (b) ?00? adjust range ?0? adjust range 12 11 10 r l1 , r l2 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 r lin , r s and r z to achieve best fit 4 to 20ma output. r z is used to set 4ma at minimum input, r s is adjusted for 12ma with a half span input, and r lin 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. r lin will range between 500 w and 1500 w for 100 w sensors (pt100, d100, sama). initially it may seem a little strange adjusting r s for 12ma and r lin 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. 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. zero adjust (optional) o s1 , o s2 , o s3 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 v os drift errors that occur when the trim is performed at the input stage. if possible use r z to trim sensor output error to zero and use the offset control to trim the output to 4ma when v in = 0v. the offset adjustment can be made with a figure 2. basic connection for zero adjust. table i. r lin values for pt100 sensor. t min ( c) 50 100 200 300 400 500 600 700 800 900 1000 C200 573 653 839 995 1083 1131 1152 1159 1159 1154 1140 C150 745 855 1059 1158 1197 1206 1205 1196 1175 1151 1127 C100 983 1105 1228 1251 1249 1231 1207 1182 1156 1129 C50 1233 1284 1286 1262 1236 1208 1180 1152 1125 1097 0 1302 1287 1273 1229 1201 1173 1145 1117 1089 50 1263 1249 1220 1192 1164 1136 1108 1081 1054 100 1225 1211 1183 1155 1127 1100 1073 1046 150 1188 1174 1146 1119 1091 1064 1038 1011 200 1151 1137 1110 1083 1056 1030 1003 250 1114 1101 1074 1048 1021 995 969 300 1079 1066 1039 1013 987 962 350 1044 1031 1005 979 954 928 400 1009 996 971 946 921 450 975 963 938 913 888 500 942 930 905 881 550 909 897 873 849 600 877 865 841 650 845 834 810 700 814 803 750 784 773 800 754 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 100 w nominal sensors. (3) double r lin value for pt200. span d t ( c)
IXR100 6 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. the transient conduction of charge during and after the ionization can be detected externally as a burst of 0.01 m s-0.1 m s 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. 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. 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. 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 eleva- tion or suppression. see the signal suppression and elevation section for the proper techniques. com this is the return for the two excitation currents i ref1 and i ref2 and is the reference point for the inputs. v s , i out these are the connections for the current loop v s being the most positive connection. for correct operation these pins should have 11.6 to 36v between them. 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 high- voltage stress across every devices isolation barrier. no partial discharge may be initiated to pass this test. this criterion confirms transient overvoltage (1.6 x v rated ) protection without damage. life-test results verify the ab- sence of failure under continuous rated voltage and maxi- mum temperature. this new test method represents the state-of-the-art for nondestructive high voltage reliability testing. it is based on figure 3. transient and rfi protection circuit. 4 1 3 2 c 1 + IXR100 5 0.4ma tranzorb r 1 r z rtd note: (1) r and r should be made equal if used (?% resistors are adequate). 12 0.01? 3.9k w r 2 c bypass c bypass optional input filtering (1) (1) = transmitter case 0.4ma 7 6 s 9 8 lin r r 1n4148 c bypass 28 18 r l + v out v s
IXR100 7 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 trans- ducer 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 r 1 and r 2 are 100-1000 w , and for c 1 are 100-1000pf. 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 turn- on. 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 high- energy transients such as those induced by lightning or motor contactors. choose a tranzorb with a voltage rating close to, but exceeding, the maximum v s which the IXR100 will see. in combination, the coaxial bypass capaci- tors and tranzorb provide a very high level of protec- tion against transients and rfi. input bandwidth limiting filtering at the input to the IXR100 is recommended where possible and can be done as shown in figure 4. c 1 connected to pins 3 and 4 will reduce the bandwidth with a f C3db frequency given by: f C3db = 0.159/(r 1 + r 2 + rtd + r z ) (c 1 + 3pf) this method has the disadvantage of having f C3db vary with r 1 , r 2 , rtd, and r z may require large values of r 1 , and r 2 . r 1 and r 2 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 suppres- sion). 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. 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 IXR100s output offset current. major points to consider when using the IXR100 1. the leads to r s and r lin should be kept as short as possible to reduce noise pick-up and parasitic resistance. if the linearity correction feature is not desired, the r lin pins are left open. 2. +v s should be bypassed with a 0.01 m f capacitor as close to the unit as possible (pins 18 to 28). 3. always keep the input voltages within their range of linear operation, +2v to +4v ( v in measured with respect to pin 5). figure 4. optional bandwidth-limiting circuitry. figure 5. elevation and suppression graph. 0 i (ma) 0 20 15 10 5 0 + elevated zero range suppressed zero range span adjust v in 4 1 3 2 c 1 + IXR100 5 0.4ma r 1 r 2 r z rtd note: (1) r and r should be made equal if used. 12 0.01? 3.9k w (1) (1) 0.4ma 7 6 s 9 8 lin r r
IXR100 8 figure 6. elevation and suppression circuits. 4. the maximum input signal level ( d v in ) is 1v with r s open and is less as r s decreases in value. 5. always return the current references to com (pin 5) through an appropriate value of r cm to keep v cm within its operating range. also, operate the current sources within their rated compliance voltage: v in + v iref (com + 7v) 6. always choose r l , (including line resistance) so that the voltage between pins 18 and 28 (+v s ) 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 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. 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. 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 incorporat- ing 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. v in e' 2 + 0.4ma 0.4ma + r 4 v 4 0.8ma r t e' 2 + 0.4ma 0.4 ma + r 4 v 4 0.8ma r t + + e' 2 + 0.8ma + r 4 v 4 0.8ma + + 0.8ma + r 4 v 4 0.8ma + e' 2 v in v in v in v in = (e' 2 Cv 4 ) v 4 = 0.8ma x r 4 (c) elevated zero range v in = (e' 2 +v 4 ) v 4 = 0.8ma x r 4 (d) suppressed zero range v in = (e' 2 Cv 4 ) v 4 = 0.4ma x r 4 e' 2 = 0.4ma x r t (a) elevated zero range v in = (e' 2 +v 4 ) v 4 = 0.4ma x r 4 e' 2 = 0.4ma x r t (b) suppressed zero range
IXR100 9 5. control relative humidity to as high a value as practical (50% recommended). rtd applications the IXR100 has been designed with rtd applications specifically in mind. the following information provides additional information for those applications. two- and three-wire connections the IXR100 performs well with two-wire and three-wire rtd connections commonly encountered in industrial moni- toring and control. in two-wire applications, the voltage drop between the rtd and the IXR100 can be nulled by proper adjustment of r z , 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 correc- tion 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 large- gauge wires. this limitation does not apply for three-wire connections. in three-wire applications, shown in figure 7, the rtd and r z 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 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 connec- tions such as screw terminals do not violate this assumption by introducing unequal line resistances. rtd zero elevation and suppression the IXR100 may be operated in zero-elevated and zero- suppressed ranges by simply offsetting r z . it may also be used in increase-decrease applications by interchanging the physical locations of the rtd and r z as shown in figure 8. use the same values of r z , r lin and r s . again, because the current sources are matched and are modulated equally, this connection has no effect on IXR100 performance, especially in three-wire applications. 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. terminal open i out (1) 1 32ma 2 3.6ma 3 32ma note: (1) approximate value. figure 7. basic 3-wire rtd connection for increase-increase action. + 4 1 3 7 6 2 r s v in + IXR100 r = 3.9k w cm 5 0.4ma 0.01? 0.4ma d 1 three-wire connection r z rtd 1 3 2 9 8 r lin 1n4148 r l + v out +v s 4-20ma 28 18
IXR100 10 other applications 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 trans- ducer transmitter modules and by data acquisition systems manufacturers who gather transducer data. current mode transmission greatly reduces noise interference. the two- wire nature of the device allows economical signal condi- tioning 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. example 1 pt100 rtd without linearization shown in figure 9. 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 109.73 w at 25 c and 157.31 w 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 47.6 w . when excited with a 0.4ma current source d v in is 0.4ma x 47.6 w = 19mv. 0.016/( d v in ) C 0.016 figure 8. basic 3-wire rtd connection for increase-decrease action. r s = w (1) from equation (1), r s = 48.5 w . span adjustment (calibra- tion) is accomplished by trimming r s . 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. v in must be 0v at 25 c and r z is chosen to be equal to the rtd resistance at 25 c, or 109.73 w . computing r cm and checking cmv: at +25 c, v in+ = 43.9mv at +150 c, v in+ = 62.9mv since both v in+ and v z are small relative to the desired 2v common-mode voltage, they may be ignored in computing r cm as long as the cmv is met. r cm = 3v/0.8ma = 3.75k w v in+ min = 3v + 0.0439v v in+ max = 3v + 0.0629v v inC = 3v + 0.0439v example 2 thermocouple shown in figure 10. given a process with temperature (t l ) 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 thermo- couple with its cold junction (t 2 ) at ambient. at +25 c this is 1.28mv (from thermocouple tables with reference junction at 0 c). typically, at t 2 = +25 c, v d = 0.6v and 40 + 3 7 6 2 r s v in + IXR100 r = 3.9k w cm 0.4ma 0.01? 0.4ma 1 three-wire connection rtd 1 3 2 r z 9 8 r lin d 1n4148 5 r l + v out +v s 4-20ma 4 1 28 18
IXR100 11 d v d / d t = C2mv/ c. r 5 and r 6 form a voltage divider for the diode voltage v d . the divider values are selected so that the gradient d v d / d t equals the gradient of the thermo- couple at the reference temperature. at +25 c this is approximately C52 m v/ c (obtained from standard thermo- couple table); therefore, d v tc / d t = ( d v d / d t)(r 6 /(r 5 + r 6 )) (2) C52 m v/ c = (C2000 m v/ c)(r 6 /(r 5 +r 6 )) r 5 is chosen as 3.74k w to be much larger than the resistance of the diode. solving for r 6 yields 100 w . transmit 4ma for t l = 0 c and 20ma for t l = +1000 c. note: v ln = v in+ C v inC indicates that t l is relative to t 2 . the input full scale span is 58mv. r s is found from equation (1) and equals 153.9 w . r 4 is chosen to make the output 4ma at t tc = 0 c (v tc = 1.28mv) and t d = 25 c (v d = 0.6v). v tc will be C1.28mv when t tc = 0 c and the reference junction is at +25 c. v 4 must be computed for t d = +25 c to make v in = 0v. v d(25 c) = 600mv v in(25 c) = 600mv (100/3740) = 16.0mv v in = v in+ C v inC = v tc + v 4 C v inC with v in = 0 and v tc = C1.28mv, v 4 = v in+ C v tc v 4 = 16.0mv C (C1.28mv) 0.4ma (r 4 ) = 17.28mv r 4 = 43.2 w 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 thermo- couple gets very large (open) the bias current flowing into the + input (large impedance) will cause i o 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 t c opens, the output will go to its upper range limit value (about 32ma or higher). figure 9. pt100 rtd without linearization. 3 7 6 2 r s v in + IXR100 + 5 0.4ma r z rtd 0.4ma r cm 0.01? d 1 1n4148 r l + v out +v s 4-20ma 4 1 28 18
IXR100 12 figure 11. thermocouple input with diode cold junction compensation and down scale burn-out indication. figure 10. thermocouple input circuit with two temperature regions and diode (d) cold junction compensation. + r s + IXR100 3.9k w 0.4ma 50 w 100 w zero adjust 0.4ma this circuit has down scale burn-out indication. type j 3.9k w 3 7 6 2 5 0.01? 1 d 1n4148 r l + v out +v s 4-20ma 4 1 28 18 +v s + IXR100 0.4ma 0.4ma 100 w + 3.9k w 3.74k w r 5 r 6 v in+ 3 5 2 r 4 v 4 0.01? v tc thermocouple t temperature t 1 tc temperature t = t d 2 + i out 7 6 r s 153.9 w + + 1n4148 43.2 w v d + v in 4 1 28 18
IXR100 13 figure 14. isolated 4-20ma instrument loop. figure 12. thermocouple input with rtd cold junction compensation and down scale burn-out indication. figure 13. thermocouple input with rtd cold junction compensation and up scale burn-out indication. r s + IXR100 3.9k w 0.4ma rtd 100 w 50 w zero 51 w + 0.4ma this circuit has up scale burn-out indication. zero adjust 3 7 6 2 5 type j 0.01? 1 d 1n4148 r l + v out +v s 4-20ma 4 1 28 18 + r s + IXR100 3.9k w 0.4ma rtd 100 w 50 w zero adjust 51 w 0.4ma this circuit has down scale burn-out indication. type j 3 7 6 2 5 0.01? 1 d 1n4148 r l + v out +v s 4-20ma 4 1 28 18 18 IXR100 5 14v to 38v ? s 1? 5 4 + 2 3 15 13 14 11 10 12 8 9 + r lin r s r z 0.4ma 0.4ma 7 6 3 2 3.9k w 0.01? 1? v out 0 - 5v rcv420 16 + +v s rtd 1n4148 4 1 28
IXR100 14 figure 15. 4-20ma to 0-20ma output converter. IXR100 opa177 ref200 400 w i in r 1 r 2 100 w r 3 19.5k w 0.01? ? s 100? v in 2 3 4-20ma note: (1) other conversions are readily achievable by changing the r 1 , r 2 , and r 3 ratios (see burr-brown application bulletin ab-031). i o = i in (i + ) + i r (r 1 + r 2 + r 3 )/r 1 = 1.25i in ?5ma (1) r r 2 1 0-20ma i o +v s ? s i r com 1 4

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