4-20mA CURRENT TRANSMITTER with Bridge Excitation and Linearization

Transcription

1 -ma CURRENT TRANSMITTER with Bridge Excitation and Linearization FEATURES LOW TOTAL UNADJUSTED ERROR.V, V BRIDGE EXCITATION REFERENCE.ULATOR OUTPUT LOW SPAN DRIFT: ±ppm/ C max LOW OFFSET DRIFT:.µV/ C HIGH PSR: db min HIGH CMR: db min WIDE SUPPLY RANGE:.V to V -PIN DIP AND SO- SURFACE-MOUNT DESCRIPTION The is a low cost, monolithic -ma, twowire current transmitter designed for bridge sensors. It provides complete bridge excitation (.V or V reference), instrumentation amplifier, sensor linearization, and current output circuitry. Current for powering additional external input circuitry is available from the pin. The instrumentation amplifier can be used over a wide range of gain, accommodating a variety of input signal types and sensors. Total unadjusted error of the complete current transmitter, including the linearized bridge, is low enough to permit use without adjustment in many applications. The operates on loop power supply voltages down to.v. Linearization circuitry provides second-order correction to the transfer function by controlling bridge excitation voltage. It provides up to a : improvement in nonlinearity, even with low cost transducers. The is available in -pin plastic DIP and SO- surface-mount packages and is specified for the C to C temperature range. Operation is from C to C. V APPLICATIONS PRESSURE BRIDGE TRANSMITTER STRAIN GAGE TRANSMITTER TEMPERATURE BRIDGE TRANSMITTER INDUSTRIAL PROCESS CONTROL SCADA REMOTE DATA ACQUISITION REMOTE TRANSDUCERS WEIGHING SYSTEMS ACCELEROMETERS Nonlinearity (%)..... mv V REF BRIDGE NONLINEARITY CORRECTION USING V REF.V (.V).V to V Corrected mv Bridge Output R LIN Uncorrected Bridge Output -ma mv V PS V O R L Lin I OUT International Airport Industrial Park Mailing Address: PO Box, Tucson, AZ Street Address: S. Tucson Blvd., Tucson, AZ Tel: () - Twx: 9-9- Internet: FAXLine: () - (US/Canada Only) Cable: BBRCORP Telex: -9 FAX: () 9- Immediate Product Info: () - 99 Burr-Brown Corporation PDS-9A Printed in U.S.A. June, 99 SBOS9

2 SPECIFICATIONS At T A = C, V = V, and TIP9C external transistor, unless otherwise noted. P, U PA, UA PARAMETER CONDITIONS MIN TYP MAX MIN TYP MAX UNITS OUTPUT Output Current Equation I O I O = (/ ) ma, in Volts, in Ω A Output Current, Specified Range ma Over-Scale Limit I OVER ma Under-Scale Limit I UNDER I REG =, I REF =.. ma I REF I REG =.ma.9. ma ZERO OUTPUT () I ZERO = V, = ma Initial Error ± ± ± µa vs Temperature T A = C to C ±. ±.9 µa/ C vs Supply Voltage, V V =.V to V.. µa/v vs Common-Mode Voltage (CMRR) V CM =.V to.v (). µa/v vs (I O ). µa/ma Noise:.Hz to Hz i n. µap-p SPAN Span Equation (Transconductance) S S = / A/V Untrimmed Error Full Scale ( ) = mv ±. ±. ±. % vs Temperature () T A = C to C ± ± ppm/ C Nonlinearity: Ideal Input () Full Scale ( ) = mv ±. ±. % INPUT () Offset Voltage V OS V CM =.V ± ± ± µv vs Temperature T A = C to C ±. ±. ± µv/ C vs Supply Voltage, V V =.V to V ±. ± µv/v vs Common-Mode Voltage, RTI CMRR V CM =.V to.v () ± ± ± µv/v Common-Mode Range () V CM.. V Input Bias Current I B na vs Temperature T A = C to C pa/ C Input Offset Current I OS ±. ± ± na vs Temperature T A = C to C pa/ C Impedance: Differential Z IN. GΩ pf Common-Mode GΩ pf Noise:.Hz to Hz V n. µvp-p VOLTAGE REFERENCES () Lin Connected to, R LIN = Initial:.V Reference V REF.. V V Reference V REF V Accuracy V REF =.V or V ±. ±. ±. % vs Temperature T A = C to C ± ± ± ppm/ C vs Supply Voltage, V V =.V to V ± ± ppm/v vs Load I REF = ma to.ma ppm/ma Noise:.Hz to Hz µvp-p V () REG. V Accuracy ±. ±. V vs Temperature T A = C to C ±. mv/ C vs Supply Voltage, V V =.V to V mv/v Output Current I REG See Typical Curves ma Output Impedance I REG = ma to.ma Ω LINEARIZATION () B R LIN (external) Equation R LIN R LIN = K LIN, K LIN in Ω, B is nonlinearity relative to V B FS Ω K LIN Linearization Factor K LIN V REF = V. kω V REF =.V 9.9 kω Accuracy ± ± % vs Temperature T A = C to C ± ± ppm/ C Max Correctable Sensor Nonlinearity B V REF = V ± % of V FS V REF =.V., % of V FS POWER SUPPLY V Specified V Voltage Range. V TEMPERATURE RANGE Specification C Operating C Storage C Thermal Resistance θ JA -Pin DIP C/W SO- Surface Mount C/W Specification same as P, U. NOTES: () Describes accuracy of the ma low-scale offset current. Does not include input amplifier effects. Can be trimmed to zero. () Does not include initial error or TCR of gain-setting resistor,. () Increasing the full-scale input range improves nonlinearity. () Does not include Zero Output initial error. () Voltage measured with respect to pin. () See Linearization text for detailed explanation. V FS = full-scale.

3 PIN CONFIGURATION ABSOLUTE MAXIMUM RATINGS () Top View V IN DIP and SOIC V REF V REF. Lin Power Supply, V (referenced to I O pin)... V Input Voltage,, (referenced to pin)... V to V Storage Temperature Range... C to C Lead Temperature (soldering, s)... C Output Current Limit... Continuous Junction Temperature... C NOTE: () Stresses above these ratings may cause permanent damage. Exposure to absolute maximum conditions for extended periods may degrade device reliability. 9 R LIN V B (Base) ELECTROSTATIC DISCHARGE SENSITIVITY I O E (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 () RANGE MARKING NUMBER () MEDIA P -Pin DIP C to C P P Rails PA -Pin DIP C to C PA PA Rails U SO- Surface Mount C to C U U Rails " " " " " U/K Tape and Reel UA SO- Surface Mount C to C UA UA Rails " " " " " UA/K Tape and Reel NOTES: () For detailed drawing and dimension table, please see end of data sheet, or Appendix C of Burr-Brown IC Data Book. () Models with a slash (/) are available only in Tape and Reel in the quantities indicated (e.g., /K indicates devices per reel). Ordering pieces of U/K will get a single -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.

4 FUNCTIONAL DIAGRAM Lin R LIN V V REF V REF. REF Amp Bandgap V REF Lin Amp.V Current Direction Switch µa B 9 9Ω Ω I = µa E I O = ma ( )

5 TYPICAL PERFORMANCE CURVES At T A = C, V = V, unless otherwise noted. Transconductance ( log ma/v) TRANSCONDUCTANCE vs FREQUENCY C OUT =.µf = Ω C OUT =.µf C OUT connected between V and I O = kω R L = Ω ma/div ma ma = kω STEP RESPONSE C OUT =.µf = Ω k k k M Frequency (Hz) µs/div Common-Mode Rejection (db) 9 COMMON-MODE REJECTION vs FREQUENCY = kω = Ω Power Supply Rejection (db) POWER SUPPLY REJECTION vs FREQUENCY C OUT = = Ω = kω k k k M Frequency (Hz) k k k M Frequency (Hz) Percent of Units (%) 9 INPUT OFFSET VOLTAGE DRIFT PRODUCTION DISTRIBUTION Typical production distribution of packaged units. V OS (µv) INPUT OFFSET VOLTAGE CHANGE vs and V REF CURRENTS V OS vs I REG V OS vs I REF Current (ma) Offset Voltage Drift (µv/ C)

6 TYPICAL PERFORMANCE CURVES (CONT) At T A = C, V = V, unless otherwise noted.. UNDER-SCALE CURRENT vs TEMPERATURE. UNDER-SCALE CURRENT vs I REF I REG Under-Scale Current (ma).... V =.V to V Under-Scale Current (ma) T A = C T A = C T A = C Temperature ( C)..... I REF I REG (ma) Over-Scale Current (ma) 9 OVER-SCALE CURRENT vs TEMPERATURE With External Transistor V = V V =.V V = V Zero Output Error (µa) ZERO OUTPUT ERROR vs V REF and CURRENTS I ZERO Error vs I REG I ZERO Error vs I REF Temperature ( C) Current (ma). Zero Output Current Error (µa) ZERO OUTPUT CURRENT ERROR vs TEMPERATURE Percent of Units (%) ZERO OUTPUT DRIFT PRODUCTION DISTRIBUTION Typical production distribution of packaged units. Temperature ( C) Zero Output Drift (µa/ C)

7 TYPICAL PERFORMANCE CURVES (CONT) At T A = C, V = V, unless otherwise noted. k INPUT VOLTAGE, INPUT CURRENT, and ZERO OUTPUT CURRENT NOISE DENSITY vs FREQUENCY k INPUT BIAS and OFFSET CURRENT vs TEMPERATURE Input Voltage Noise (nv/ Hz) k Zero Output Noise Input Current Noise Input Voltage Noise k k Frequency (Hz) k k Input Current Noise (fa/ Hz) Zero Output Current Noise (pa/ Hz) Input Bias and Offset Current (na) I B I OS Temperature ( C). OUTPUT VOLTAGE vs OUTPUT CURRENT REFERENCE TRANSIENT RESPONSE V REF = V Output Current (V) T A = C, C T A = C µa/div mv/div Reference Output ma Output Current (ma) µs/div. V REF vs OUTPUT CURRENT REFERENCE AC LINE REJECTION vs FREQUENCY V REF (V). T A = C..99 T A = C.99 T A = C Current (ma) Line Rejection (db) V REF. V REF k k k M Frequency (Hz)

8 TYPICAL PERFORMANCE CURVES (CONT) At T A = C, V = V, unless otherwise noted. Percent of Units (%) REFERENCE VOLTAGE DRIFT PRODUCTION DISTRIBUTION Typical production distribution of packaged units. Reference Voltage Drift (ppm/ C) Reference Voltage Deviation (%)..... REFERENCE VOLTAGE DEVIATION vs TEMPERATURE V REF =.V V REF = V. Temperature ( C)

9 APPLICATIONS INFORMATION Figure shows the basic connection diagram for the. The loop power supply, V PS, provides power for all circuitry. Output loop current is measured as a voltage across the series load resistor, R L. A.µF to.µf supply bypass capacitor connected between V and I O is recommended. For applications where fault and/or overload conditions might saturate the inputs, a.µf capacitor is recommended. A.V or V reference is available to excite a bridge sensor. For V excitation, pin (V REF ) should be connected to the bridge as shown in Figure. For.V excitation, connect pin (V REF.) to pin as shown in Figure b. The output terminals of the bridge are connected to the instrumentation amplifier inputs, and. A.µF capacitor is shown connected between the inputs and is recommended for high impedance bridges (> kω). The resistor sets the gain of the instrumentation amplifier as required by the full-scale bridge voltage, V FS. Lin and R LIN provide second-order linearization correction to the bridge, achieving up to a : improvement in linearity. Connections to Lin (pin ) determine the polarity of nonlinearity correction and should be connected either to or. Lin should be connected to even if linearity correction is not desired. R LIN is chosen according to the equation in Figure and is dependent on K LIN (linearization constant) and the bridge s nonlinearity relative to V FS (see Linearization section). The transfer function for the complete current transmitter is: I O = ma (/ ) () in Volts, in Ohms where is the differential input voltage. As evident from the transfer function, if no is used ( = ), the gain is zero and the output is simply the s zero current. A negative input voltage,, will cause the output current to be less than ma. Increasingly negative will cause the output current to limit at approximately.ma. If current is being sourced from the reference and/or, the current limit value may increase. Refer to the Typical Performance Curves, Under-Scale Current vs I REF I REG and Under- Scale Current vs Temperature. Increasingly positive input voltage (greater than the fullscale input, V FS ) will produce increasing output current according to the transfer function, up to the output current limit of approximately ma. Refer to the Typical Performance Curve, Over-Scale Current vs Temperature. The pin is the return path for all current from the references and. also serves as a local ground and is the reference point for and the on-board voltage references. The pin allows any current used in external circuitry to be sensed by the and to be included in the output current without causing error. The input voltage range of the is referred to this pin. For.V excitation, connect pin to pin V REF V REF. R LIN () R LIN VREG V.V to V Possible choices for Q (see text). TYPE N9 TIP9C TIPC PACKAGE TO- TO- TO- R () () R Bridge Sensor V R B C IN.µF () () Lin () I O B E 9 Q C OUT.µF I O - ma V O R L V PS I = ma ( O ) V () REG or NOTES: () Connect Lin (pin ) to (pin ) to correct for positive bridge nonlinearity or connect to (pin ) for negative bridge nonlinearity. The R LIN pin and Lin pin must be connected to if linearity correction is not desired. Refer to Linearization section and Figure. () Recommended for bridge impedances > kω B ( ) R LIN = K LIN (K LIN in Ω) B FIGURE. Basic Bridge Measurement Circuit with Linearization. 9 () = (V FS /µa) B B (V FS in V) where K LIN = 9.9kΩ for.v reference K LIN =.kω for V reference B is the bridge nonlinearity relative to V FS V FS is the full-scale input voltage () R and R form bridge trim circuit to compensate for the initial accuracy of the bridge. See Bridge Balance text.

10 EXTERNAL TRANSISTOR External pass transistor, Q, conducts the majority of the signal-dependent -ma loop current. Using an external transistor isolates the majority of the power dissipation from the precision input and reference circuitry of the, maintaining excellent accuracy. Since the external transistor is inside a feedback loop its characteristics are not critical. Requirements are: V CEO = V min, β = min and P D = mw. Power dissipation requirements may be lower if the loop power supply voltage is less than V. Some possible choices for Q are listed in Figure. The can be operated without an external pass transistor. Accuracy, however, will be somewhat degraded due to the internal power dissipation. Operation without Q is not recommended for extended temperature ranges. A resistor (R =.kω) connected between the pin and the E (emitter) pin may be needed for operation below C without Q to guarantee the full ma full-scale output, especially with V near.v. LOOP POWER SUPPLY The voltage applied to the, V, is measured with respect to the I O connection, pin. V can range from.v to V. The loop supply voltage, V PS, will differ from the voltage applied to the according to the voltage drop on the current sensing resistor, R L (plus any other voltage drop in the line). If a low loop supply voltage is used, R L (including the loop wiring resistance) must be made a relatively low value to assure that V remains.v or greater for the maximum loop current of ma: () R L max = (V).V ma R WIRING It is recommended to design for V equal or greater than.v with loop currents up to ma to allow for out-ofrange input conditions. V must be at least V if V sensor excitation is used and if correcting for bridge nonlinearity greater than %. V I O E R Q =.kω.µf For operation without external transistor, connect a.kω resistor between pin and pin. See text for discussion of performance. FIGURE. Operation without External Transistor. The low operating voltage (.V) of the allows operation directly from personal computer power supplies (V ±%). When used with the RCV Current Loop Receiver (Figure ), load resistor voltage drop is limited to V. BRIDGE BALANCE Figure shows a bridge trim circuit (R, R ). This adjustment can be used to compensate for the initial accuracy of the bridge and/or to trim the offset voltage of the. The values of R and R depend on the impedance of the bridge, and the trim range required. This trim circuit places an additional load on the V REF output. Be sure the additional load on V REF does not affect zero output. See the Typical Performance Curve, Under-Scale Current vs I REF I REG. The effective load of the trim circuit is nearly equal to R. An approximate value for R can be calculated: R V R () B V TRIM where, R B is the resistance of the bridge. V TRIM is the desired ±voltage trim range (in V). Make R equal or lower in value to R. LINEARIZATION Many bridge sensors are inherently nonlinear. With the addition of one external resistor, it is possible to compensate for parabolic nonlinearity resulting in up to : improvement over an uncompensated bridge output. Linearity correction is accomplished by varying the bridge excitation voltage. Signal-dependent variation of the bridge excitation voltage adds a second-order term to the overall transfer function (including the bridge). This can be tailored to correct for bridge sensor nonlinearity. Either positive or negative bridge non-linearity errors can be compensated by proper connection of the Lin pin. To correct for positive bridge nonlinearity (upward bowing), Lin (pin ) should be connected to (pin ) as shown in Figure a. This causes V REF to increase with bridge output which compensates for a positive bow in the bridge response. To correct negative nonlinearity (downward bowing), connect Lin to (pin ) as shown in Figure b. This causes V REF to decrease with bridge output. The Lin pin is a high impedance node. If no linearity correction is desired, both the R LIN and Lin pins should be connected to (Figure c). This results in a constant reference voltage independent of input signal. R LIN or Lin pins should not be left open or connected to another potential. R LIN is the external linearization resistor and is connected between pin and pin ( ) as shown in Figures a and b. To determine the value of R LIN, the nonlinearity of the bridge sensor with constant excitation voltage must be known. The s linearity circuitry can only compensate for the parabolic-shaped portions of a sensor s nonlinearity. Optimum correction occurs when maximum deviation from linear output occurs at mid-scale (see Figure ). Sensors with nonlinearity curves similar to that shown in

11 Figure, but not peaking exactly at mid-scale can be substantially improved. A sensor with a S-shaped nonlinearity curve (equal positive and negative nonlinearity) cannot be improved with the s correction circuitry. The value of R LIN is chosen according to Equation shown in Figure. R LIN is dependent on a linearization factor, K LIN, which differs for the.v reference and V reference. The sensor s nonlinearity term, B (relative to full scale), is positive or negative depending on the direction of the bow. A maximum ±% non-linearity can be corrected when the V reference is used. Sensor nonlinearity of %/.% can be corrected with.v excitation. The trim circuit shown in Figure d can be used for bridges with unknown bridge nonlinearity polarity. Gain is affected by the varying excitation voltage used to correct bridge nonlinearity. The corrected value of the gain resistor is calculated from Equation given in Figure. V REF V REF. V R LIN Lin R R.V a. Connection for Positive Bridge Nonlinearity, V REF = V R R V REF. V REF b. Connection for Negative Bridge Nonlinearity, V REF =.V R R V V REF V REF. Lin R LIN Lin R LIN R LIN = R X R Y kω kω Open R X for negative bridge nonlinearity Open R Y for positive bridge nonlinearity d. On-Board Resistor Circuit for Unknown Bridge Nonlinearity EQUATIONS Linearization Resistor: B R LIN = K LIN B Gain-Set Resistor: = V FS µa B B Adjusted Excitation Voltage at Full-Scale Output: V REF (Adj) = V REF (Initial) B (in V) B (99Ω) () (.) () (.) (in Ω) (in Ω) where, K LIN is the linearization factor (in Ω) K LIN = 99Ω for the.v reference K LIN = Ω for the V reference B is the sensor nonlinearity relative to V FS (for.% nonlinearity, B =.) V FS is the full-scale bridge output without linearization (in V) Example: Calculate R LIN and the resulting for a bridge sensor with.% downward bow nonlinearity relative to V FS and determine if the input common-mode range is valid. V REF =.V and V FS = mv For a.% downward bow, B =. (Lin pin connected to ) For V REF =.V, K LIN = 99Ω = 9Ω () () () Lin =.V () (.) µa () (.) = Ω V CM = V REF (Adj) =.V () (.) () (.) =.V c. Connection if no linearity correction is desired, V REF = V which falls within the.v to.v input common-mode range. FIGURE. Connections and Equations to Correct Positive and Negative Bridge Nonlinearity.

12 When using linearity correction, care should be taken to insure that the sensor s output common-mode voltage remains within the s allowable input range of.v to.v. Equation in Figure can be used to calculate the s new excitation voltage. The common-mode voltage of the bridge output is simply half this value if no common-mode resistor is used (refer to the example in Figure ). Exceeding the common-mode range may yield unpredicatable results. For high precision applications (errors < %), a two-step calibration process can be employed. First, the nonlinearity of the sensor bridge is measured with the initial gain resistor and R LIN = (R LIN pin connected directly to ). Using the resulting sensor nonlinearity, B, values for and R LIN are calculated using Equations and from Figure. A second calibration measurement is then taken to adjust to account for the offsets and mismatches in the linearization. UNDER-SCALE CURRENT The total current being drawn from the V REF and voltage sources, as well as temperature, affect the s under-scale current value (see the Typical Performance Curve, Under-Scale Current vs I REF I REG ). This should be considered when choosing the bridge resistance and excitation voltage, especially for transducers operating over a wide temperature range (see the Typical Performance Curve, Under-Scale Current vs Temperature ). LOW IMPEDANCE BRIDGES The s two available excitation voltages (.V and V) allow the use of a wide variety of bridge values. Bridge impedances as low as kω can be used without any additional circuitry. Lower impedance bridges can be used with the by adding a series resistance to limit excitation current to.ma (Figure ). Resistance should be added BRIDGE TRANSDUCER TRANSFER FUNCTION WITH PARABOLIC NONLINEARITY NONLINEARITY vs STIMULUS 9 Bridge Output (mv) Positive Nonlinearity B =. B =.9 Negative Nonlinearity Linear Response Nonlinearity (% of Full Scale) Positive Nonlinearity B =. Negative Nonlinearity B = Normalized Stimulus Normalized Stimulus FIGURE. Parabolic Nonlinearity. µa at V V REF I TOTAL =.ma.ma.ma I REG.mA V REF..kΩ V / OPA kω V IN R LIN V N Ω.kΩ Ω / OPA kω kω Ω V IN B 9 E Lin I O.µF I O = -ma Bridge excitation voltage =.V Approx. x amplifier Shown connected to correct positive bridge nonlinearity. For negative bridge nonlinearity, see Figure b. FIGURE. Ω Bridge with x Preamplifier.

13 to the upper and lower sides of the bridge to keep the bridge output within the.v to.v common-mode input range. Bridge output is reduced so a preamplifier as shown may be needed to reduce offset voltage and drift. OTHER SENSOR TYPES The can be used with a wide variety of inputs. Its high input impedance instrumentation amplifier is versatile and can be configured for differential input voltages from millivolts to a maximum of.v full scale. The linear range of the inputs is from.v to.v, referenced to the terminal, pin. The linearization feature of the can be used with any sensor whose output is ratiometric with an excitation voltage. ERROR ANALYSIS Table I shows how to calculate the effect various error sources have on circuit accuracy. A sample error calculation for a typical bridge sensor measurement circuit is shown (kω bridge, V REF = V, V FS = mv) is provided. The results reveal the s excellent accuracy, in this case.% unadjusted. Adjusting gain and offset errors improves circuit accuracy to.%. Note that these are worst-case errors; guaranteed maximum values were used in the calculations and all errors were assumed to be positive (additive). The achieves performance which is difficult to obtain with discrete circuitry and requires less board space. SAMPLE ERROR CALCULATION Bridge Impedance (R B ) kω Full Scale Input (V FS ) mv Ambient Temperature Range ( T A ) C Excitation Voltage (V REF ) V Supply Voltage Change ( V) V Common-Mode Voltage Change ( CM) mv (= V FS /) ERROR SAMPLE (ppm of Full Scale) ERROR SOURCE ERROR EQUATION ERROR CALCULATION UNADJ ADJUST INPUT Input Offset Voltage V OS /V FS µv/mv vs Common-Mode CMRR CM/V FS µv/v.v/mv vs Power Supply (V OS vs V) ( V)/V FS µv/v V/mV Input Bias Current CMRR I B (R B /)/ V FS µv/v na.kω/mv. Input Offset Current I OS R B /V FS na kω/mv Total Input Error EXCITATION Voltage Reference Accuracy V REF Accuracy (%)/%.%/% vs Supply (V REF vs V) ( V) (V FS /V REF ) ppm/v V (mv/v) Total Excitation Error GAIN Span Span Error (%)/%.%/% Nonlinearity Nonlinearity (%)/%.%/% Total Gain Error OUTPUT Zero Output I ZERO ma /µa µa/µa vs Supply (I ZERO vs V) ( V)/µA.µA/V V/µA.. Total Output Error DRIFT ( T A = C) Input Offset Voltage Drift T A /(V FS ).µv/ C C / (mv) Input Offset Current (typical) Drift T A R B /(V FS ) pa / C C kω/ (mv) Voltage Refrence Accuracy ppm/ C C Span ppm/ C C Zero Output Drift T A / µa.9µa/ C C / µa Total Drift Error 9 9 NOISE (.Hz to Hz, typ) Input Offset Voltage V n (p-p)/ V FS.µV / mv Zero Output I ZERO Noise / µa.µa / µa.. Thermal R B Noise [ (R B /)/kω nv / Hz Hz ] / V FS [.kω /kω nv/ Hz Hz ] / mv.. Input Current Noise (i n. R B /)/V FS (fa/ Hz..kΩ)/mV.. Total Noise Error NOTE (): All errors are min/max and referred to input, unless otherwise stated. TOTAL ERROR:.%.% TABLE I. Error Calculation.

14 REVERSE-VOLTAGE PROTECTION The s low compliance rating (.V) permits the use of various voltage protection methods without compromising operating range. Figure shows a diode bridge circuit which allows normal operation even when the voltage connection lines are reversed. The bridge causes a two diode drop (approximately.v) loss in loop supply voltage. This results in a compliance voltage of approximately 9V satisfactory for most applications. A diode can be inserted in series with the loop supply voltage and the V pin as shown in Figure to protect against reverse output connection lines with only a.v loss in loop supply voltage. OVER-VOLTAGE SURGE PROTECTION Remote connections to current transmitters can sometimes be subjected to voltage surges. It is prudent to limit the maximum surge voltage applied to the to as low as practical. Various zener diode and surge clamping diodes are specially designed for this purpose. Select a clamp diode with as low a voltage rating as possible for best protection. For example, a V protection diode will assure proper transmitter operation at normal loop voltages, yet will provide an appropriate level of protection against voltage surges. Characterization tests on three production lots showed no damage to the with loop supply voltages up to V. Most surge protection zener diodes have a diode characteristic in the forward direction that will conduct excessive current, possibly damaging receiving-side circuitry if the loop connections are reversed. If a surge protection diode is used, a series diode or diode bridge should be used for protection against reversed connections. RADIO FREQUENCY INTERFERENCE The long wire lengths of current loops invite radio frequency interference. RF can be rectified by the sensitive input circuitry of the causing errors. This generally appears as an unstable output current that varies with the position of loop supply or input wiring. If the bridge sensor is remotely located, the interference may enter at the input terminals. For integrated transmitter assemblies with short connection to the sensor, the interference more likely comes from the current loop connections. Bypass capacitors on the input reduce or eliminate this input interference. Connect these bypass capacitors to the terminal as shown in Figure. Although the dc voltage at the terminal is not equal to V (at the loop supply, V PS ) this circuit point can be considered the transmitter s ground. The.µF capacitor connected between V and I O may help minimize output interference. V REF V REF. V V Maximum V PS must be less than minimum voltage rating of zener diode. R B B 9 Q.µF D () N Diodes Bridge Sensor.µF.µF I O E The diode bridge causes a.v loss in loop supply voltage. R L V PS NOTE: () Zener Diode V: NA or Motorola PKE9A. Use lower voltage zener diodes with loop power supply voltages less than V for increased protection. See Over-Voltage Surge Protection. FIGURE. Reverse Voltage Operation and Over-Voltage Surge Protection.

15 V REF.µF See ISO data sheet if isolation is needed. kω.kω MΩ V REF. Isothermal Block kω OPA R LIN V.V to V Type K N Ω.kΩ Ω MΩ () kω kω 9 B Q C OUT.µF E Lin I O I O = ma ( ) (pin ) I O - ma V O R L V PS NOTE: () For burn-out indication..µf FIGURE. Thermocouple Low Offset, Low Drift Loop Measurement with Diode Cold-Junction Compensation. Bridge Sensor.V R B V REF V REF. R LIN V N V µf See ISO data sheet if isolation is needed. Lin B E I O 9 I O = -ma.µf RCV µf V O = V to V NOTE: Lin shown connected to correct positive bridge nonlinearity. See Figure b to correct negative bridge nonlinearity. V FIGURE. ±V-Powered Transmitter/Receiver Loop.

16 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, Texas Instruments Incorporated