Download: 4-20mA CURRENT TRANSMITTER with Bridge Excitation and Linearization FEATURES APPLICATIONS ● LOW TOTAL UNADJUSTED ERROR ● PRESSURE BRIDGE TRANSMITTER ● 2.5V, 5V BRIDGE EXCITATION REFERENCE ● STRAIN GAGE TRANSMITTER

® XTR106 XTR106 XTR106 4-20mA CURRENT TRANSMITTER with Bridge Excitation and Linearization

FEATURES APPLICATIONS

● LOW TOTAL UNADJUSTED ERROR ● PRESSURE BRIDGE TRANSMITTER ● 2.5V, 5V BRIDGE EXCITATION REFERENCE ● STRAIN GAGE TRANSMITTER ● 5.1V REGULATOR OUTPUT ● TEMPERATURE BRIDGE TRANSMITTER ● LOW SPAN DRIFT: ±25ppm/°C max ● INDUSTRIAL PROCESS CONTROL ● LOW OFFSET DRIFT: 0.25µV/°C ● SCADA REMOTE DATA ACQUISITION ● HIGH PSR: 110dB min ● REMOTE TRANSDUCERS ● HIGH CMR: 86dB min ● WEIGHING SYSTEMS ● WIDE SUPPLY RANGE: 7.5V to 36V ● ACCELEROMETERS ● 14-PIN DIP AND SO-14 SURFACE-MOUNT BRIDGE NONLINEARITY CORRECTION USING XTR106

DESCRIPTION 2.0 Uncorrected

Bridge Output The XTR106 is a low cost, monolithic 4-20mA, two- 1.5 wire current transmitter designed for bridge sensors. It provides complete bridge excitation (2.5V or 5V refer- 1.0 ence), instrumentation amplifier, sensor linearization, and current output circuitry. Current for powering ad- 0.5 ditional external input circuitry is available from the V CorrectedREG pin. 0 The instrumentation amplifier can be used over a wide range of gain, accommodating a variety of input signal –0.5 types and sensors. Total unadjusted error of the com- 0mV 5mV 10mV plete current transmitter, including the linearized bridge, Bridge Output is low enough to permit use without adjustment in many applications. The XTR106 operates on loop power sup- VREG (5.1V) ply voltages down to 7.5V. V 5 VREF 2.5V Linearization circuitry provides second-order correction REF to the transfer function by controlling bridge excitation RLIN voltage. It provides up to a 20:1 improvement in + 7.5V to 36V nonlinearity, even with low cost transducers. VPS The XTR106 is available in 14-pin plastic DIP and 4-20mA SO-14 surface-mount packages and is specified for the 5V R XTR106 VG O –40°C to +85°C temperature range. Operation is from –55°C to +125°C. RL – Lin IOUT Polarity

IRET

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SPECIFICATIONS

At TA = +25°C, V+ = 24V, and TIP29C external transistor, unless otherwise noted. XTR106P, U XTR106PA, UA PARAMETER CONDITIONS MIN TYP MAX MIN TYP MAX UNITS

OUTPUT

Output Current Equation IO IO = VIN • (40/RG) + 4mA, VIN in Volts, RG inΩAOutput Current, Specified Range 4 20 ✻ ✻ mA Over-Scale Limit IOVER 24 28 30 ✻ ✻ ✻ mA Under-Scale Limit IUNDER IREG = 0, IREF = 0 1 1.6 2.2 ✻ ✻ ✻ mA IREF + IREG = 2.5mA 2.9 3.4 4 ✻ ✻ ✻ mA ZERO OUTPUT(1) IZERO VIN = 0V, RG = ∞ 4 ✻ mA Initial Error ±5 ±25 ✻ ±50 µA vs Temperature TA = –40°C to +85°C ±0.07 ±0.9 ✻ ✻ µA/°C vs Supply Voltage, V+ V+ = 7.5V to 36V 0.04 0.2 ✻ ✻ µA/V vs Common-Mode Voltage (CMRR) V (5)CM = 1.1V to 3.5V 0.02 ✻ µA/V vs VREG (IO) 0.8 ✻ µA/mA Noise: 0.1Hz to 10Hz in 0.035 ✻ µAp-p

SPAN

Span Equation (Transconductance) S S = 40/RG ✻ A/V Untrimmed Error Full Scale (VIN) = 50mV ±0.05 ±0.2 ✻ ±0.4 % vs Temperature(2) TA = –40°C to +85°C ±3 ±25 ✻ ✻ ppm/°C Nonlinearity: Ideal Input (3) Full Scale (VIN) = 50mV ±0.001 ±0.01 ✻ ✻ % INPUT(4) Offset Voltage VOS VCM = 2.5V ±50 ±100 ✻ ±250 µV vs Temperature TA = –40°C to +85°C ±0.25 ±1.5 ✻ ±3 µV/°C vs Supply Voltage, V+ V+ = 7.5V to 36V ±0.1 ±3 ✻ ✻ µV/V vs Common-Mode Voltage, RTI CMRR VCM = 1.1V to 3.5V(5) ±10 ±50 ✻ ±100 µV/V Common-Mode Range(5) VCM 1.1 3.5 ✻ ✻ V Input Bias Current IB 5 25 ✻ 50 nA vs Temperature TA = –40°C to +85°C 20 ✻ pA/°C Input Offset Current IOS ±0.2 ±3 ✻ ±10 nA vs Temperature TA = –40°C to +85°C 5 ✻ pA/°C Impedance: Differential ZIN 0.1 || 1 ✻ GΩ || pF Common-Mode 5 || 10 ✻ GΩ || pF Noise: 0.1Hz to 10Hz Vn 0.6 ✻ µVp-p VOLTAGE REFERENCES(5) Lin Polarity Connected to VREG, RLIN = 0 Initial: 2.5V Reference VREF2.5 2.5 ✻ V 5V Reference VREF5 5 ✻ V Accuracy VREF = 2.5V or 5V ±0.05 ±0.25 ✻ ±0.5 % vs Temperature TA = –40°C to +85°C ±20 ±35 ✻ ±75 ppm/°C vs Supply Voltage, V+ V+ = 7.5V to 36V ±5 ±20 ✻ ✻ ppm/V vs Load IREF = 0mA to 2.5mA 60 ✻ ppm/mA Noise: 0.1Hz to 10Hz 10 ✻ µVp-p V (5)REG VREG 5.1 ✻ V Accuracy ±0.02 ±0.1 ✻ ✻ V vs Temperature TA = –40°C to +85°C ±0.3 ✻ mV/°C vs Supply Voltage, V+ V+ = 7.5V to 36V 1 ✻ mV/V Output Current IREG See Typical Curves ✻ mA Output Impedance IREG = 0mA to 2.5mA 80 ✻ Ω LINEARIZATION(6) RLIN (external) Equation RLIN RLIN = K 4B LIN • , KLIN in Ω, B is nonlinearity relative to V Ω1 – 2B FS KLIN Linearization Factor KLIN VREF = 5V 6.645 ✻ kΩ VREF = 2.5V 9.905 ✻ kΩ Accuracy ±1 ±5 ✻ ✻ % vs Temperature TA = –40°C to +85°C ±50 ±100 ✻ ✻ ppm/°C Max Correctable Sensor Nonlinearity B VREF = 5V ±5 ✻ % of VFS VREF = 2.5V –2.5, +5 ✻ % of VFS POWER SUPPLY V+ Specified +24 ✻ V Voltage Range +7.5 +36 ✻ ✻ V TEMPERATURE RANGE Specification –40 +85 ✻ ✻ °C Operating –55 +125 ✻ ✻ °C Storage –55 +125 ✻ ✻ °C Thermal Resistance θJA 14-Pin DIP 80 ✻ °C/W SO-14 Surface Mount 100 ✻ °C/W ✻ Specification same as XTR106P, XTR106U. NOTES: (1) Describes accuracy of the 4mA low-scale offset current. Does not include input amplifier effects. Can be trimmed to zero. (2) Does not include initial error or TCR of gain-setting resistor, RG. (3) Increasing the full-scale input range improves nonlinearity. (4) Does not include Zero Output initial error. (5) Voltage measured with respect to IRET pin. (6) See “Linearization” text for detailed explanation. VFS = full-scale VIN. ®

XTR106 2

,

PIN CONFIGURATION ABSOLUTE MAXIMUM RATINGS(1)

Top View DIP and SOIC Power Supply, V+ (referenced to IO pin) ... 40V Input Voltage, V+ , V–IN IN (referenced to IRET pin) ... 0V to V+ Storage Temperature Range ... –55°C to +125°C Lead Temperature (soldering, 10s) ... +300°C Output Current Limit ... Continuous 1 Junction Temperature ... +165°CVREG 14 VREF5 – NOTE: (1) Stresses above these ratings may cause permanent damage.VIN 2 13 VREF2.5 Exposure to absolute maximum conditions for extended periods may degradeR312 Lin Polarity device reliability.G RG 4 11 RLIN V +IN 5 10 V+

ELECTROSTATIC

I DISCHARGE SENSITIVITYRET69B(Base) IO78E(Emitter) This integrated circuit can be damaged by ESD. Burr-Brown recommends that all integrated circuits be handled with appropriate precautions. Failure to observe proper handling and installation procedures can cause damage.

ESD damage can range from subtle performance degradation

to complete device failure. Precision integrated circuits may be more susceptible to damage because very small parametric changes could cause the device not to meet its published specifications.

PACKAGE/ORDERING INFORMATION

PACKAGE SPECIFIED DRAWING TEMPERATURE PACKAGE ORDERING TRANSPORT PRODUCT PACKAGE NUMBER(1) RANGE MARKING NUMBER(2) MEDIA XTR106P 14-Pin DIP 010 –40°C to +85°C XTR106P XTR106P Rails XTR106PA 14-Pin DIP 010 –40°C to +85°C XTR106PA XTR106PA Rails XTR106U SO-14 Surface Mount 235 –40°C to +85°C XTR106U XTR106U Rails " " " " " XTR106U/2K5 Tape and Reel XTR106UA SO-14 Surface Mount 235 –40°C to +85°C XTR106UA XTR106UA Rails " " " " " XTR106UA/2K5 Tape and Reel NOTES: (1) For detailed drawing and dimension table, please see end of data sheet, or Appendix C of Burr-Brown IC Data Book. (2) Models with a slash (/) are available only in Tape and Reel in the quantities indicated (e.g., /2K5 indicates 2500 devices per reel). Ordering 2500 pieces of “XTR106U/2K5” will get a single 2500-piece Tape and Reel. For detailed Tape and Reel mechanical information, refer to Appendix B 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. ® 3 XTR106,

FUNCTIONAL DIAGRAM VREG

Lin Polarity 12 RLIN V+ V REFREF5 Amp Bandgap VREF 5.1V VREF2.5 Lin Amp Current Direction Switch V+IN 4 100µA

B RG

975Ω 25Ω

E V

2 I = 100µA + IN 8

R

V – GIN 6 40 IO = 4mA + VIN • ( ) RG

IRET

®

XTR106 4

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TYPICAL PERFORMANCE CURVES

At TA = +25°C, V+ = 24V, unless otherwise noted. TRANSCONDUCTANCE vs FREQUENCY STEP RESPONSE CCOOUUTT = = 0 0.0.011µµFF COUT = 0.01µFRG = 50Ω C 50 OUT = 0.033µF RG = 1kΩ COUT connected 20mA between V+ and I 40 O RG = 50Ω 30 RG = 1kΩ 4mA RL = 250Ω 100 1k 10k 100k 1M 50µs/div Frequency (Hz) COMMON-MODE REJECTION vs FREQUENCY POWER SUPPLY REJECTION vs FREQUENCY 110 160 C = 0 RG = 50Ω OUT100 140 90 120 80 R = 50Ω 100 RG = 1kΩG 70 RG = 1kΩ 80 60 60 50 40 40 20 30 0 10 100 1k 10k 100k 1M 10 100 1k 10k 100k 1M Frequency (Hz) Frequency (Hz) INPUT OFFSET VOLTAGE DRIFT INPUT OFFSET VOLTAGE CHANGE PRODUCTION DISTRIBUTION vs VREG and VREF CURRENTS 90 1.5 Typical production 80 1.0 VOS vs Idistribution of REG 70 packaged units. 0.5 –0.5 40 VOS vs IREF –1.0 –1.5 10 –2.0 0 –2.5 –1.0 –0.5 0 0.5 1.0 1.5 2.0 2.5 Current (mA) Offset Voltage Drift (µV/°C) ® 5 XTR106 Common-Mode Rejection (dB) Percent of Units (%) Transconductance (20 log mA/V) 0.25 0.5 0.75 1.0 1.25 1.5 1.75 2.0 2.25 2.5 2.75 3.0 ∆ VOS (µV) Power Supply Rejection (dB) 4mA/div,

TYPICAL PERFORMANCE CURVES (CONT)

At TA = +25°C, V+ = 24V, unless otherwise noted. UNDER-SCALE CURRENT vs TEMPERATURE UNDER-SCALE CURRENT vs IREF + IREG 2.5 4.0 3.5 2.0 3.0 TA = –55°C 1.5 2.5 2.0 TA = +25°C 1.0 1.5 1.0 TA = +125°C 0.5 V+ = 7.5V to 36V 0.500–75 –50 –25 0 25 50 75 100 125 0 0.5 1.0 1.5 2.0 2.5 Temperature (°C) IREF + IREG (mA) ZERO OUTPUT ERROR OVER-SCALE CURRENT vs TEMPERATURE vs VREF and VREG CURRENTS 30 3.0 With External Transistor 2.5 2.0 IZERO Error vs IREG 1.5 V+ = 36V 27 1.0 V+ = 7.5V 0.5 26 IZERO Error vs IREF V+ = 24V 0 –0.5 24 –1.0 –75 –50 –25 0 25 50 75 100 125 –1 –0.5 0 0.5 1.0 1.5 2.0 2.5 Temperature (°C) Current (mA) ZERO OUTPUT CURRENT ERROR ZERO OUTPUT DRIFT vs TEMPERATURE PRODUCTION DISTRIBUTION 4 70 Typical production 2 60 distribution of packaged units. –2 –4 –6 –8 –10 10 –12 0 –75 –50 –25 0 25 50 75 100 125 Temperature (°C) Zero Output Drift (µA/°C) ®

XTR106 6

Zero Output Current Error (µA) Over-Scale Current (mA) Under-Scale Current (mA) Zero Output Error (µA) Under-Scale Current (mA) Percent of Units (%) 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9,

TYPICAL PERFORMANCE CURVES (CONT)

At TA = +25°C, V+ = 24V, unless otherwise noted. INPUT VOLTAGE, INPUT CURRENT, and ZERO INPUT BIAS and OFFSET CURRENT OUTPUT CURRENT NOISE DENSITY vs FREQUENCY vs TEMPERATURE 10k 10k 10 Zero Output Noise 1k 1k 6 IB Input Current Noise 100 100 2

IOS

Input Voltage Noise 10 10 –2 1 10 100 1k 10k 100k –75 –50 –25 0 25 50 75 100 125 Frequency (Hz) Temperature (°C) REFERENCE TRANSIENT RESPONSE VREG OUTPUT VOLTAGE vs VREG OUTPUT CURRENT VREF = 5V 5.6 5.5 5.4 5.3 5.2 TA = +25°C, –55°C 5.1 1mA 5.0 4.9 TA = +125°C 0 4.8 –1.0 –0.5 0 0.5 1.0 1.5 2.0 2.5 10µs/div VREG Output Current (mA) VREF5 vs VREG OUTPUT CURRENT REFERENCE AC LINE REJECTION vs FREQUENCY 5.008 120 5.004 TA = +25°C 80 VREF2.5 5.000V54.996 REF TA = +125°C 4.992 20 TA = –55°C 4.988 0 –1.0 –0.5 0 0.5 1.0 1.5 2.0 2.5 10 100 1k 10k 100k 1M VREG Current (mA) Frequency (Hz) ® 7 XTR106 VREF5 (V) VREG Output Current (V) Input Voltage Noise (nV/√Hz) Input Current Noise (fA/√Hz) Zero Output Current Noise (pA/√Hz) Line Rejection (dB) Input Bias and Offset Current (nA) 500µA/div 50mV/div Reference Output,

TYPICAL PERFORMANCE CURVES (CONT)

At TA = +25°C, V+ = 24V, unless otherwise noted. REFERENCE VOLTAGE DEVIATION REFERENCE VOLTAGE DRIFT vs TEMPERATURE PRODUCTION DISTRIBUTION 40 0.1 Typical production 35 distribution of 0 packaged units. –0.1 25 VREF = 5V 20 –0.2 VREF = 2.5V 15 –0.3 –0.4 0 –0.5 –75 –50 –25 0 25 50 75 100 125 Temperature (°C) Reference Voltage Drift (ppm/°C) ®

XTR106 8

Percent of Units (%) Reference Voltage Deviation (%),

APPLICATIONS INFORMATION The transfer function for the complete current transmitter is:

I = 4mA + V • (40/R ) (1) Figure 1 shows the basic connection diagram for the XTR106. O IN G The loop power supply, VPS, provides power for all cir- VIN in Volts, RG in Ohms cuitry. Output loop current is measured as a voltage across where VIN is the differential input voltage. As evident from the series load resistor, RL. A 0.01µF to 0.03µF supply the transfer function, if no RG is used (RG = ∞), the gain is bypass capacitor connected between V+ and IO is recom- zero and the output is simply the XTR106’s zero current. mended. For applications where fault and/or overload con- µ A negative input voltage, VIN, will cause the output currentditions might saturate the inputs, a 0.03 F capacitor is to be less than 4mA. Increasingly negative VIN will cause therecommended. output current to limit at approximately 1.6mA. If current is A 2.5V or 5V reference is available to excite a bridge sensor. being sourced from the reference and/or VREG, the current For 5V excitation, pin 14 (VREF5) should be connected to the limit value may increase. Refer to the Typical Performance bridge as shown in Figure 1. For 2.5V excitation, connect Curves, “Under-Scale Current vs IREF + IREG” and “Under- pin 13 (VREF2.5) to pin 14 as shown in Figure 3b. The output Scale Current vs Temperature.” terminals of the bridge are connected to the instrumentation + – µ Increasingly positive input voltage (greater than the full-amplifier inputs, VIN and VIN. A 0.01 F capacitor is shown scale input, V ) will produce increasing output current connected between the inputs and is recommended for high FS Ω according to the transfer function, up to the output currentimpedance bridges (> 10k ). The resistor RG sets the gain limit of approximately 28mA. Refer to the Typical Perfor- of the instrumentation amplifier as required by the full-scale mance Curve, “Over-Scale Current vs Temperature.” bridge voltage, VFS. The I pin is the return path for all current from the Lin Polarity and RLIN provide second-order linearization

RET

references and V . I also serves as a local ground and correction to the bridge, achieving up to a 20:1 improvement REG RET is the reference point for V and the on-board voltage in linearity. Connections to Lin Polarity (pin 12) determine REG references. The I pin allows any current used in external the polarity of nonlinearity correction and should be con- RET circuitry to be sensed by the XTR106 and to be included in nected either to IRET or VREG. Lin Polarity should be con- the output current without causing error. The input voltage nected to VREG even if linearity correction is not desired. range of the XTR106 is referred to this pin. RLIN is chosen according to the equation in Figure 1 and is dependent on KLIN (linearization constant) and the bridge’s nonlinearity relative to VFS (see “Linearization” section). For 2.5V excitation, connect VREG Possible choices for Q1 (see text). pin 13 to pin 14 TYPE PACKAGE VREF5 VREF2.5 R (3)LIN 2N4922 TO-225 TIP29C TO-220 14 TIP31C TO-220 5 11 7.5V to 36V + R 1VIN LIN V 10REG V+ 5V C

IO

IN 4 (5) 0.01µF (2) RG 4-20 mA R1 C (5) (4) 9B OUTR2 + RB – R XTR106 QG 1 0.01µF

VO

Bridge 3 +R E SensorG8RL VPS Lin(1) IO – V–2 PolarityIN 7 IRET 126IO= 4mA + VIN • ( 4 0 ) V (1) RGREG or NOTES: 1 + 2B (4) (V in V) (1) Connect Lin Polarity (pin 12) to IRET (pin 6) to correct for positive RG = (VFS/400µA) • FS 1 – 2B bridge nonlinearity or connect to VREG (pin 1) for negative bridge nonlinearity. The RLIN pin and Lin Polarity pin must be connected to where KLIN = 9.905kΩ for 2.5V reference VREG if linearity correction is not desired. Refer to “Linearization” KLIN = 6.645kΩ for 5V reference section and Figure 3. B is the bridge nonlinearity relative to VFS (2) Recommended for bridge impedances > 10kΩ VFS is the full-scale input voltage 4B (5) R1 and R2 form bridge trim circuit to compensate for the initial( 3)RLIN = KLIN • (KLIN in Ω)1 – 2B accuracy of the bridge. See “Bridge Balance” text. FIGURE 1. Basic Bridge Measurement Circuit with Linearization. ® 9 XTR106, EXTERNAL TRANSISTOR The low operating voltage (7.5V) of the XTR106 allows External pass transistor, Q1, conducts the majority of the operation directly from personal computer power supplies signal-dependent 4-20mA loop current. Using an external (12V ±5%). When used with the RCV420 Current Loop transistor isolates the majority of the power dissipation from Receiver (Figure 8), load resistor voltage drop is limited to 3V. the precision input and reference circuitry of the XTR106, maintaining excellent accuracy. BRIDGE BALANCE Since the external transistor is inside a feedback loop its Figure 1 shows a bridge trim circuit (R1, R2). This adjust- characteristics are not critical. Requirements are: VCEO = 45V ment can be used to compensate for the initial accuracy of min, β = 40 min and PD = 800mW. Power dissipation require- the bridge and/or to trim the offset voltage of the XTR106. ments may be lower if the loop power supply voltage is less The values of R1 and R2 depend on the impedance of the than 36V. Some possible choices for Q1 are listed in Figure 1. bridge, and the trim range required. This trim circuit places The XTR106 can be operated without an external pass an additional load on the VREF output. Be sure the additional transistor. Accuracy, however, will be somewhat degraded load on VREF does not affect zero output. See the Typical due to the internal power dissipation. Operation without Q Performance Curve, “Under-Scale Current vs IREF + IREG.”1 is not recommended for extended temperature ranges. A The effective load of the trim circuit is nearly equal to R2. resistor (R = 3.3kΩ) connected between the IRET pin and the An approximate value for R1 can be calculated: E (emitter) pin may be needed for operation below 0°C 5V • R (3)B without Q1 to guarantee the full 20mA full-scale output, R1 ≈ 4 • VTRIM especially with V+ near 7.5V. where, RB is the resistance of the bridge. VTRIM is the desired ±voltage trim range (in V). Make R2 equal or lower in value to R1.

LINEARIZATION

V+ Many bridge sensors are inherently nonlinear. With the addition of one external resistor, it is possible to compensate E for parabolic nonlinearity resulting in up to 20:1 improve- XTR106 0.01µF ment over an uncompensated bridge output. Linearity correction is accomplished by varying the bridge IO excitation voltage. Signal-dependent variation of the bridge 7 excitation voltage adds a second-order term to the overall IRET For operation without external transfer function (including the bridge). This can be tailored 6 transistor, connect a 3.3kΩ resistor between pin 6 and to correct for bridge sensor nonlinearity.RQ = 3.3kΩ pin 8. See text for discussion Either positive or negative bridge non-linearity errors can be of performance. compensated by proper connection of the Lin Polarity pin. FIGURE 2. Operation without External Transistor. To correct for positive bridge nonlinearity (upward bowing), Lin Polarity (pin 12) should be connected to IRET (pin 6) as LOOP POWER SUPPLY shown in Figure 3a. This causes VREF to increase with bridge The voltage applied to the XTR106, V+, is measured with output which compensates for a positive bow in the bridge respect to the I connection, pin 7. V+ can range from 7.5V response. To correct negative nonlinearity (downward bow-O to 36V. The loop supply voltage, V , will differ from the ing), connect Lin Polarity to VREG (pin 1) as shown in FigurePS voltage applied to the XTR106 according to the voltage drop 3b. This causes VREF to decrease with bridge output. The Lin on the current sensing resistor, R (plus any other voltage Polarity pin is a high impedance node.L drop in the line). If no linearity correction is desired, both the RLIN and Lin If a low loop supply voltage is used, R (including the loop Polarity pins should be connected to VREG (Figure 3c). ThisL wiring resistance) must be made a relatively low value to results in a constant reference voltage independent of input assure that V+ remains 7.5V or greater for the maximum signal. RLIN or Lin Polarity pins should not be left open loop current of 20mA: or connected to another potential. (2) RLIN is the external linearization resistor and is connected(V+) – 7.5V R max = L – RWIRING between pin 11 and pin 1 (VREG) as shown in Figures 3a and 20mA 3b. To determine the value of RLIN, the nonlinearity of the It is recommended to design for V+ equal or greater than bridge sensor with constant excitation voltage must be 7.5V with loop currents up to 30mA to allow for out-of- known. The XTR106’s linearity circuitry can only compen- range input conditions. V+ must be at least 8V if 5V sensor sate for the parabolic-shaped portions of a sensor’s excitation is used and if correcting for bridge nonlinearity nonlinearity. Optimum correction occurs when maximum greater than +3%. deviation from linear output occurs at mid-scale (see Figure 4). Sensors with nonlinearity curves similar to that shown in ®

XTR106 10

,

Figure 4, but not peaking exactly at mid-scale can be A maximum ±5% non-linearity can be corrected when the

substantially improved. A sensor with a “S-shaped” 5V reference is used. Sensor nonlinearity of +5%/–2.5% can nonlinearity curve (equal positive and negative nonlinearity) be corrected with 2.5V excitation. The trim circuit shown in cannot be improved with the XTR106’s correction circuitry. Figure 3d can be used for bridges with unknown bridge

The value of RLIN is chosen according to Equation 4 shown

nonlinearity polarity. in Figure 3. RLIN is dependent on a linearization factor, Gain is affected by the varying excitation voltage used to

KLIN, which differs for the 2.5V reference and 5V reference. correct bridge nonlinearity. The corrected value of the gain The sensor’s nonlinearity term, B (relative to full scale), is resistor is calculated from Equation 5 given in Figure 3.

positive or negative depending on the direction of the bow.

VREG

VREF5 XTR106VREF2.5 Lin VREG 14 R 13 LIN Polarity151+ 11 IRET 12 5V46R1 R2 + – RG XTR106 RX RY 100kΩ 15kΩ 3 Open RX for negative bridge nonlinearity Open RY for positive bridge nonlinearity – 3d. On-Board Resistor Circuit for Unknown Bridge Nonlinearity Polarity 12 Lin 6 Polarity IRET EQUATIONS 3a. Connection for Positive Bridge Nonlinearity, VREF = 5V Linearization Resistor: V (4)REG 4BRLIN = KLIN • (in Ω) V 1– 2BREF2.5 VREF5 Gain-Set Resistor: 13 RLIN VFS 1 + 2B (5) 5 1 RG = • (in Ω) + 11 400µA 1 – 2B 2.5V 4 Adjusted Excitation Voltage at Full-Scale Output: R1R1+ 2B (6)2 + – R VXTR106 REF (Adj) = VREF (Initial) • (in V)G 1 – 2B where, KLIN is the linearization factor (in Ω) 2 KLIN = 9905Ω for the 2.5V reference – KLIN = 6645Ω for the 5V reference 12 Lin Polarity B is the sensor nonlinearity relative to VFS6 (for –2.5% nonlinearity, B = –0.025)

IRET

VFS is the full-scale bridge output without 3b. Connection for Negative Bridge Nonlinearity, V = 2.5V linearization (in V)REF V Example:REG VREF5 Calculate RLIN and the resulting RG for a bridge sensor with VREF2.5 2.5% downward bow nonlinearity relative to VFS and determine if the input common-mode range is valid. 13 RLIN VREF = 2.5V and VFS = 50mV51+ 11 For a 2.5% downward bow, B = –0.025 5V 4 (Lin Polarity pin connected to VREG) R For VREF = 2.5V, KLIN = 9905Ω1 R2 + – RG XTR106 (9905Ω) (4) (–0.025) RLIN = = 943Ω31– (2) (–0.025) 2 0.05V 1 + (2) (–0.025) – RG = • = 113Ω400µA 1 – (2) (–0.025) 12 Lin 6 Polarity V= REF (Adj) = 1 • • 1 + (2) (–0.025)I VCM 2.5V = 1.13VRET221– (2) (–0.025) 3c. Connection if no linearity correction is desired, VREF = 5V which falls within the 1.1V to 3.5V input common-mode range.

FIGURE 3. Connections and Equations to Correct Positive and Negative Bridge Nonlinearity.

® 11 XTR106, When using linearity correction, care should be taken to UNDER-SCALE CURRENT insure that the sensor’s output common-mode voltage re- The total current being drawn from the V and V mains within the XTR106’s allowable input range of 1.1V to REF REGvoltage sources, as well as temperature, affect the XTR106’s 3.5V. Equation 6 in Figure 3 can be used to calculate the under-scale current value (see the Typical Performance XTR106’s new excitation voltage. The common-mode volt- Curve, “Under-Scale Current vs I + I ). This should be age of the bridge output is simply half this value if no REF REGconsidered when choosing the bridge resistance and excita- common-mode resistor is used (refer to the example in tion voltage, especially for transducers operating over a Figure 3). Exceeding the common-mode range may yield wide temperature range (see the Typical Performance Curve, unpredicatable results. “Under-Scale Current vs Temperature”). For high precision applications (errors < 1%), a two-step calibration process can be employed. First, the nonlinearity LOW IMPEDANCE BRIDGES of the sensor bridge is measured with the initial gain resistor and R = 0 (R pin connected directly to V ). Using The XTR106’s two available excitation voltages (2.5V andLIN LIN REG the resulting sensor nonlinearity, B, values for RG and R 5V) allow the use of a wide variety of bridge values. Bridge

LIN

are calculated using Equations 4 and 5 from Figure 3. A impedances as low as 1kΩ can be used without any addi- second calibration measurement is then taken to adjust R to tional circuitry. Lower impedance bridges can be used withG account for the offsets and mismatches in the linearization. the XTR106 by adding a series resistance to limit excitation current to ≤ 2.5mA (Figure 5). Resistance should be added BRIDGE TRANSDUCER TRANSFER FUNCTION WITH PARABOLIC NONLINEARITY NONLINEARITY vs STIMULUS 10382Positive Nonlinearity 7 Positive Nonlinearity B = +0.025 B = +0.025150B= –0.019 3 –1Negative Nonlinearity 2 Linear Response –2 1 Negative Nonlinearity B = –0.019 0 –3 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9100.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Normalized Stimulus Normalized Stimulus FIGURE 4. Parabolic Nonlinearity. 700µA at 5V VREF5 ITOTAL = 0.7mA + 1.6mA ≤ 2.5mA IREG ≈ 1.6mA VREF2.5 VREG 3.4kΩ 13 RLIN 1N4148 1kΩ 1/2 5 11+ 5V OPA2277 V 10IN V+ 10kΩ RG 350Ω RGB9125Ω XTR106 0.01µF 412Ω 10kΩ3REG 8 Lin IO V– Polarity 3.4kΩ IN 71/2 2 IRET 12 OPA2277 6 IO = 4-20mA Shown connected to correct positive Bridge excitation Approx. x50 bridge nonlinearity. For negative bridge voltage = 0.245V amplifier nonlinearity, see Figure 3b. FIGURE 5. 350Ω Bridge with x50 Preamplifier. ®

XTR106 12

Bridge Output (mV) Nonlinearity (% of Full Scale), to the upper and lower sides of the bridge to keep the bridge ERROR ANALYSIS output within the 1.1V to 3.5V common-mode input range. Table I shows how to calculate the effect various error

Bridge output is reduced so a preamplifier as shown may be sources have on circuit accuracy. A sample error calculation

needed to reduce offset voltage and drift. for a typical bridge sensor measurement circuit is shown (5kΩ bridge, VREF = 5V, VFS = 50mV) is provided. The

OTHER SENSOR TYPES results reveal the XTR106’s excellent accuracy, in this case The XTR106 can be used with a wide variety of inputs. Its 1.2% unadjusted. Adjusting gain and offset errors improves

high input impedance instrumentation amplifier is versatile circuit accuracy to 0.33%. Note that these are worst-case and can be configured for differential input voltages from errors; guaranteed maximum values were used in the calcu- millivolts to a maximum of 2.4V full scale. The linear range lations and all errors were assumed to be positive (additive). of the inputs is from 1.1V to 3.5V, referenced to the I The XTR106 achieves performance which is difficult toRET terminal, pin 6. The linearization feature of the XTR106 can obtain with discrete circuitry and requires less board space. be used with any sensor whose output is ratiometric with an excitation voltage. SAMPLE ERROR CALCULATION Bridge Impedance (RB) 5kΩ Full Scale Input (VFS) 50mV Ambient Temperature Range (∆TA) 20°C Excitation Voltage (VREF) 5V Supply Voltage Change (∆V+) 5V Common-Mode Voltage Change (∆CM) 25mV (= VFS/2)

ERROR

SAMPLE (ppm of Full Scale) ERROR SOURCE ERROR EQUATION ERROR CALCULATION UNADJ ADJUST

INPUT

Input Offset Voltage VOS /VFS • 106 200µV/50mV • 106 2000 0 vs Common-Mode CMRR • ∆CM/VFS • 106 50µV/V • 0.025V/50mV • 106 25 25 vs Power Supply (VOS vs V+) • (∆V+)/VFS • 106 3µV/V • 5V/50mV • 106 300 300 Input Bias Current CMRR • IB • (RB /2)/ V 6FS • 10 50µV/V • 25nA • 2.5kΩ/50mV • 106 0.1 0 Input Offset Current IOS • RB /VFS • 106 3nA • 5kΩ/50mV • 106 300 0 Total Input Error 2625 325

EXCITATION

Voltage Reference Accuracy VREF Accuracy (%)/100% • 106 0.25%/100% • 106 2500 0 vs Supply (VREF vs V+) • (∆V+) • (VFS/VREF) 20ppm/V • 5V (50mV/5V) 1 1 Total Excitation Error 2501 1

GAIN

Span Span Error (%)/100% • 106 0.2%/100% • 106 2000 0 Nonlinearity Nonlinearity (%)/100% • 106 0.01%/100% • 106 100 100 Total Gain Error 2100 100

OUTPUT

Zero Output | IZERO – 4mA | /16000µA • 106 25µA/16000µA • 106 1563 0 vs Supply (IZERO vs V+) • (∆V+)/16000µA • 106 0.2µA/V • 5V/16000µA • 106 62.5 62.5 Total Output Error 1626 63 DRIFT (∆TA = 20°C) Input Offset Voltage Drift • ∆TA / (VFS) • 106 1.5µV / °C • 20°C / (50mV) • 106 600 600 Input Offset Current (typical) Drift • ∆T • R / (V ) • 106A B FS 5pA / °C • 20°C • 5kΩ/ (50mV) • 106 10 10 Voltage Refrence Accuracy 35ppm/°C • 20°C 700 700 Span 225ppm/°C • 20°C 500 500 Zero Output Drift • ∆TA / 16000µA • 106 0.9µA /°C • 20°C / 16000µA • 106 1125 1125 Total Drift Error 2936 2936 NOISE (0.1Hz to 10Hz, typ) Input Offset Voltage Vn(p-p)/ VFS • 106 0.6µV / 50mV • 106 12 12 Zero Output IZERO Noise / 16000µA • 106 0.035µA / 16000µA • 106 2.2 2.2 Thermal RB Noise [√ 2 • √ (RB / 2 ) / 1kΩ • 4nV / √ Hz • √ 10Hz ] / V 6 6FS • 10 [√ 2 • √ 2.5kΩ / 1kΩ • 4nV/ √ Hz • √ 10Hz ] / 50mV • 10 0.6 0.6 Input Current Noise (in • 40.8 • √2 • R 6 6B / 2)/ VFS • 10 (200fA/√Hz • 40.8 • √2 • 2.5kΩ)/50mV• 10 0.6 0.6 Total Noise Error 15 15 NOTE (1): All errors are min/max and referred to input, unless otherwise stated. TOTAL ERROR: 11803 3340 1.18% 0.33%

TABLE I. Error Calculation.

® 13 XTR106, REVERSE-VOLTAGE PROTECTION Most surge protection zener diodes have a diode character- The XTR106’s low compliance rating (7.5V) permits the istic in the forward direction that will conduct excessive use of various voltage protection methods without compro- current, possibly damaging receiving-side circuitry if the mising operating range. Figure 6 shows a diode bridge loop connections are reversed. If a surge protection diode is circuit which allows normal operation even when the volt- used, a series diode or diode bridge should be used for age connection lines are reversed. The bridge causes a two protection against reversed connections. diode drop (approximately 1.4V) loss in loop supply volt- age. This results in a compliance voltage of approximately RADIO FREQUENCY INTERFERENCE 9V—satisfactory for most applications. A diode can be The long wire lengths of current loops invite radio fre- inserted in series with the loop supply voltage and the V+ quency interference. RF can be rectified by the sensitive pin as shown in Figure 8 to protect against reverse output input circuitry of the XTR106 causing errors. This generally connection lines with only a 0.7V loss in loop supply appears as an unstable output current that varies with the voltage. position of loop supply or input wiring. If the bridge sensor is remotely located, the interference may OVER-VOLTAGE SURGE PROTECTION enter at the input terminals. For integrated transmitter as- Remote connections to current transmitters can sometimes be semblies with short connection to the sensor, the interfer- subjected to voltage surges. It is prudent to limit the maximum ence more likely comes from the current loop connections. surge voltage applied to the XTR106 to as low as practical. Bypass capacitors on the input reduce or eliminate this input Various zener diode and surge clamping diodes are specially interference. Connect these bypass capacitors to the I designed for this purpose. Select a clamp diode with as low a RETterminal as shown in Figure 6. Although the dc voltage at voltage rating as possible for best protection. For example, a the I terminal is not equal to 0V (at the loop supply, V ) 36V protection diode will assure proper transmitter operation RET PSthis circuit point can be considered the transmitter’s “ground.” at normal loop voltages, yet will provide an appropriate level The 0.01µF capacitor connected between V+ and I may of protection against voltage surges. Characterization tests on Ohelp minimize output interference. three production lots showed no damage to the XTR106 with loop supply voltages up to 65V. VREF5 VREF2.5 5 + Maximum VPS must be VIN 10 less than minimum V+ voltage rating of zener 5V 4 diode.

RG

9 0.01µF + R 1N4148 B – R

B

G XTR106 Q1 D (1)1 Diodes Bridge R EG 8 RL VPS Sensor IO V–2 IN 7 The diode bridge causes IRET a 1.4V loss in loop supply voltage. 0.01µF 0.01µF NOTE: (1) Zener Diode 36V: 1N4753A or Motorola P6KE39A. Use lower voltage zener diodes with loop power supply voltages less than 30V for increased protection. See “Over-Voltage Surge Protection.” FIGURE 6. Reverse Voltage Operation and Over-Voltage Surge Protection. ®

XTR106 14

, VREF5 0.01µF See ISO124 data sheet if isolation is needed. 1MΩ VREF2.5 6kΩ 4.8kΩ Isothermal 20kΩ 14 Block 13 5 11 7.5V to 36V OPA277 V +IN R LIN 10 VREG V+ 4 IO

R

TypeKG4-20 mA 9 CRBQOUTG XTR106 1 0.01µF 1MΩ(1) 1kΩ VO 3 + R EG 8 RL VPS Lin IO – 1N4148 – 2 VIN Polarity 7 IRET 12 5.2kΩ 6 IO = 4mA + VIN • ( 4 0 )

RG

50Ω VREG (pin 1) 100Ω 2kΩ NOTE: (1) For burn-out indication. 0.01µF

FIGURE 7. Thermocouple Low Offset, Low Drift Loop Measurement with Diode Cold-Junction Compensation.

VREF2.5 VREG See ISO124 data sheet if isolation is needed. VREF52.5V Bridge 14 13 RLIN 1N4148 Sensor 1 +12V V+ 11 – 5R + IN 10B 1µF V+

RG

R B 9XTR106 0.01µF 16G 1033812RG E 15 Lin IO RCV420 V14 O = 0V to 5V 2 V– PolarityIN7213 IRET 12564I1µFO = 4-20mA NOTE: Lin Polarity shown connected to correct positive bridge nonlinearity. See Figure 3b to correct negative bridge nonlinearity. –12V

FIGURE 8. ±12V-Powered Transmitter/Receiver Loop.

® 15 XTR106, IMPORTANT NOTICE Texas Instruments and its subsidiaries (TI) reserve the right to make changes to their products or to discontinue any product or service without notice, and advise customers to obtain the latest version of relevant information to verify, before placing orders, that information being relied on is current and complete. All products are sold subject to the terms and conditions of sale supplied at the time of order acknowledgment, including those pertaining to warranty, patent infringement, and limitation of liability. TI warrants performance of its semiconductor products to the specifications applicable at the time of sale in accordance with TI’s standard warranty. Testing and other quality control techniques are utilized to the extent TI deems necessary to support this warranty. Specific testing of all parameters of each device is not necessarily performed, except those mandated by government requirements. Customers are responsible for their applications using TI components. In order to minimize risks associated with the customer’s applications, adequate design and operating safeguards must be provided by the customer to minimize inherent or procedural hazards. TI assumes no liability for applications assistance or customer product design. TI does not warrant or represent that any license, either express or implied, is granted under any patent right, copyright, mask work right, or other intellectual property right of TI covering or relating to any combination, machine, or process in which such semiconductor products or services might be or are used. TI’s publication of information regarding any third party’s products or services does not constitute TI’s approval, warranty or endorsement thereof. Copyright 2000, Texas Instruments Incorporated]

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