4-20mA Current Transmitter with RTD EXCITATION AND LINEARIZATION

Transcription

1 -ma Current Transmitter with RTD XCITATION AND LINARIZATION FATURS LSS THAN ±% TOTAL ADJUSTD RROR, C TO + C RTD XCITATION AND LINARIZATION TWO OR THR-WIR RTD OPRATION WID SUPPLY RANG: V to V HIGH PSR: db min HIGH CMR: db min DSCRIPTION The is a monolithic -ma, two-wire current transmitter designed for Platinum RTD temperature sensors. It provides complete RTD current excitation, instrumentation amplifier, linearization, and current output circuitry on a single integrated circuit. Versatile linearization circuitry provides a nd-order correction to the RTD, typically achieving a : improvement in linearity. Instrumentation amplifier gain can be configured for a wide range of temperature measurements. Total adjusted error of the complete current transmitter, including the linearized RTD is less than ±% over the full to + C operating temperature range. This includes zero drift, span drift and nonlinearity. The operates on loop power supply voltages down to V. The is available in 6-pin plastic DIP and SOL-6 surface-mount packages specified for the C to + C temperature range. Nonlinearity (%) APPLICATIONS INDUSTRIAL PROCSS CONTROL FACTORY AUTOMATION SCADA C Pt NONLINARITY CORRCTION USING Uncorrected RTD Nonlinearity Corrected Nonlinearity Process Temperature ( C) =.ma =.ma + to V - ma + C V PS V O RTD R L International Airport Industrial Park Mailing Address: PO Box, Tucson, AZ Street Address: 6 S. Tucson Blvd., Tucson, AZ 6 Tel: () 6- Twx: -- Internet: FAXLine: () -6 (US/Canada Only) Cable: BBRCORP Telex: 66-6 FAX: () - Immediate Product Info: () -6 Burr-Brown Corporation PDS- Printed in U.S.A. October, SBOS

2 SPCIFICATIONS LCTRICAL At T A = + C, = V, and N6 external transistor, unless otherwise noted. BP/BU AP/AU PARAMTR CONDITIONS MIN TYP MAX MIN TYP MAX UNITS OUTPUT Output Current quation = V IN (.6 + / ) + ma, V IN in Volts, in Ω A Total Adjusted rror () T MIN to T MAX ± ± % of FS Output Current, Specified Range ma Over-Scale Limit ma Under Scale-Limit.6. ma Full Scale Output rror V IN = V, = ± ± ± µa Noise:.Hz to khz = Ω µap-p ZRO OUTPUT () V IN =, = ma Initial rror ± ± ± µa vs Temperature ±. ±. ± µa/ C vs Supply Voltage, = V to V (). µa/v vs Common-Mode Voltage V CM = V to V (). µa/v SPAN Span quation (Transconductance) S =.6 + / A/V Untrimmed rror Ω ±. ± % vs Temperature () ± ± ± ppm/ C Nonlinearity: Ideal Input. % RTD Input Pt: C to + C. % = Ω INPUT Differential Range = V Input Voltage Range () V Common-Mode Rejection V IN = V to V () db Impedance: Differential GΩ Common-Mode. GΩ Offset Voltage ±. ±. mv vs Temperature ± ±. ± ± µv/ C vs Supply Voltage, = V to V () db Input Bias Current na vs Temperature. na/ C Input Offset Current na vs Temperature.. na/ C CURRNT SOURCS () Current. ma Accuracy ±. ±. ± % vs Temperature ± ± ± ± ppm/ C vs Power Supply, = V to V () ppm/v Compliance Voltage () (V IN ). () V Matching ±. % vs Temperature ± ± ± ppm/ C vs Power Supply, = V to V () ppm/v POWR SUPPLY Voltage Range (), V TMPRATUR RANG Specification, T MIN to T MAX C Operating C θ JA C/W Specification same as BP. NOTS: () Includes corrected Pt nonlinearity for process measurement spans greater than C, and over-temperature zero and span effects. Does not include initial offset and gain errors which are normally trimmed to zero at C. () Describes accuracy of the ma low-scale offset current. Does not include input amplifier effects. Can be trimmed to zero. () Voltage measured with respect to pin. () Does not include TCR of gain-setting resistor,. () Measured with = to disable linearization feature.

3 PIN CONFIGURATION ABSOLUT MAXIMUM RATINGS TOP VIW Zero Adjust Zero Adjust 6 Zero Adjust B (Base) Power Supply, (referenced to pin)... V Input Voltage, V + IN, V IN (referenced to pin)... V to Storage Temperature Range... C to + C Lead Temperature (soldering, s)... + C Output Current Limit... Continuous Junction Temperature C V IN + V IN 6 INT (Internal mitter) (mitter) PACKAG/ORDRING INFORMATION PACKAG DRAWING TMPRATUR PRODUCT PACKAG NUMBR () RANG LCTROSTATIC DISCHARG SNSITIVITY This integrated circuit can be damaged by SD. Burr-Brown recommends that all integrated circuits be handled with appropriate precautions. Failure to observe proper handling and installation procedures can cause damage. SD 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. AP 6-pin Plastic DIP C to + C BP 6-pin Plastic DIP C to + C AU SOL-6 Surface Mount C to + C BU SOL-6 Surface Mount C to + C NOT: () For detailed drawing and dimension table, please see end of data sheet, or Appendix C 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 TYPICAL PRFORMANC CURVS At T A = + C, = VDC, unless otherwise noted. TRANSCONDUCTANC vs FRQUNCY STP RSPONS Transconductance ( Log ma/v) 6 = Ω = Ω = Ω = kω = ma ma R S = R S = Ω ma/div k k k M µs/div COMMON-MOD RJCTION vs FRQUNCY (RTI) POWR SUPPLY RJCTION vs FRQUNCY (RTI) CMR (db) 6 G =.6A/V ( = Ω) Power Supply Rejection (db) 6 G =.6A/V ( = Ω). k k k. k k k Loop Resistance, R L (Ω ) LOOP RSISTANC vs LOOP POWR SUPPLY V () V R L max = ma Ω Operating Region Loop Power Supply Voltage, V PS (V) V OS (µv) 6 INPUT OFFST VOLTAG vs LOOP POWR SUPPLY Span = = 6mA Without external transistor R L = Ω With external transistor R L = 6Ω R L = kω R L = Ω R L = 6Ω R L = kω Loop Power Supply Voltage, V PS (V)

5 TYPICAL PRFORMANC CURVS (CONT) At T A = + C, +V = VDC, unless otherwise noted. OUTPUT CURRNT NOIS DNSITY vs FRQUNCY INPUT CURRNT NOIS DNSITY vs FRQUNCY Output Current Noise (pa/ Hz) = Input Current Noise (pa/ Hz ).. k k k.. k k k k INPUT VOLTAG NOIS DNSITY vs FRQUNCY Noise Voltage (nv/ Hz ). k k k

6 APPLICATION 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. Two matched.ma current sources drive the RTD and zero-setting resistor, R Z. The instrumentation amplifier input of the measures the voltage difference between the RTD and R Z. The value of R Z is chosen to be equal to the resistance of the RTD at the low-scale (minimum) measurement temperature. R Z can be adjusted to achieve ma output at the minimum measurement temperature to correct for input offset voltage and reference current mismatch of the. R CM provides an additional voltage drop to bias the inputs of the within their common-mode range. Resistor,, sets the gain of the instrumentation amplifier according to the desired temperature measurement range. The transfer function through the complete instrumentation amplifier and voltage-to-current converter is: = V IN (.6 + / ) + ma, (V IN in volts, in ohms, = ) where V IN is the differential input voltage. With no connected ( = ), a V to V input produces a -ma output current. With = Ω, a V to mv input produces a -ma output current. Other values for can be calculated according to the desired full-scale input voltage, V FS, with the formula in Figure. Negative input voltage, V IN, will cause the output current to be less than ma. Increasingly negative V IN will cause the output current to limit at approximately.6ma. Increasingly positive input voltage (greater than V FS ) will produce increasing output current according to the transfer function, up to the output current limit of approximately ma. XTRNAL TRANSISTOR 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 CO = 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 this external transistor by connecting pin to (see Figure ). Accuracy will be somewhat degraded by the additional internal power dissipation. This effect is most pronounced when the input stage is set for high gain (for low full-scale input voltage). The typical performance curve Input Offset Voltage vs Loop Supply Voltage describes this behavior. V IN = V + IN V IN = (RTD R Z ) Possible choices for Q (see text). TYP PACKAG =.ma =.ma + V IN (, ) 6 V + IN B Q N TIPB TIPB TO- TO- TO- + R L - ma + V PS V IN RTD (, ) R Z () I = ma + V IN (.6 + O ) NOTS: () R Z = RTD resistance at the minimum measured temperature. R CM =.kω () = V FS () See Table I for values. Ω, where V FS is Full Scale V IN. FIGUR. Basic RTD Temperature Measurement Circuit. 6

7 FIGUR. Operation Without xternal Transistor. INT For operation without external transistor, connect pin to pin. See text for discussion of performance. LOOP POWR SUPPLY The voltage applied to the,, is measured with respect to the connection, pin. 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 must be made a relatively low value to assure that remains V or greater for the maximum loop current of ma. It may, in fact, be prudent to design for equal or greater than V with loop currents up to ma to allow for out-of-range input conditions. The typical performance curve Loop Resistance vs Loop Power Supply shows the allowable sense resistor values for full-scale ma. 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. LINARIZATION On-chip linearization circuitry creates a signal-dependent variation in the two matching current sources. Both current sources are varied equally according to the following equation: V IN = =. + ( in ma, V IN in volts, in ohms) (maximum =.ma) This varying excitation provides a nd-order term to the transfer function (including the RTD) which can correct the RTD s nonlinearity. The correction is controlled by resistor which is chosen according to the desired temperature measurement range. Table I provides the, R Z and resistor values for a Pt RTD. If no linearity correction is desired, do not connect a resistor to the pins ( = ). This will cause the excitation current sources to remain a constant.ma. ADJUSTING INITIAL RRORS Most applications will require adjustment of initial errors. Offset errors can be corrected by adjustment of the zero resistor, R Z. Figure shows another way to adjust zero errors using the output current adjustment pins of the. This provides a minimum of ±µa (typically ±µa) adjustment around the initial low-scale output current. This is an output current adjustment which is independent of the input stage gain set T MIN C C C C C 6 C C C C C C / / /6 /6 / /6 / /6 / / 6 6 C 6/ 6/ 6/ 6/ 6/ 6/6 6/6 6/ 6/6 C / /6 /6 /6 /6 / / /6 C / /6 / / / /6 /6 6 C /6 / / / / /6 6 C / / / / / 66 6 C / /6 / / 6 6 C /6 / / 6 C /66 /6 6 C /6 C /6 TABL I. R Z,, and Resistor Values for Pt RTD. MASURMNT TMPRATUR SPAN T ( C) R Z / (Values are in Ω.) NOT: Values shown are for a Pt RTD. Double (x) all values for Pt.

8 kω 6 (a) ±µa typical output current adjustment range. (b) Figure, shows a three-wire RTD connection for improved accuracy with remotely located RTDs. R Z s current is routed through a third wire to the RTD. Assuming line resistance is equal in RTD lines and, this produces a small commonmode voltage which is rejected by the. OPN-CIRCUIT DTCTION The optional transistor Q in Figure provides predictable behavior with open-circuit RTD connections. It assures that if any one of the three RTD connections is broken, the s output current will go to either its high current limit ( ma) or low current limit (.6mA). This is easily detected as an out-of-range condition. kω 6 kω ±µa typical output current adjustment range. FIGUR. Low-Scale Output Current Adjustment. with. If the input stage is set for high gain (as required with narrow temperature measurement spans) the output current adjustment may not provide sufficient range. In these cases, offset can be nulled by adjusting the value of R Z. TWO-WIR AND THR-WIR RTD CONNCTIONS In Figure, the RTD can be located remotely simply by extending the two connections to the RTD. With this twowire connection to the RTD, line resistance will introduce error. This error can be partially corrected by adjusting the values of R Z,, and. RVRS-VOLTAG PROTCTION Figure shows two ways to protect against reversed output connection lines. Trade-offs in an application will determine which technique is better. D offers series protection, but causes a.v loss in loop supply voltage. This may be undesirable if can approach the V limit. Using D (without D ) has no voltage loss, but high current will flow in the loop supply if the leads are reversed. This could damage the power supply or the sense resistor, R L. A diode with a higher current rating is needed for D to withstand the highest current that could occur with reversed lines. SURG PROTCTION Long lines are subject to voltage surges which can damage semiconductor components. To avoid damage, the maximum applied voltage rating for the is V. A zener diode may be used for D (Figure 6) to clamp the voltage applied to the to a safe level. The loop power supply voltage must be lower than the voltage rating of the zener diode. qual line resistances here creates a small common-mode voltage which is rejected by. ( ) R Z V + IN Q RTD Resistance in this line causes a small common-mode voltage which is rejected by. Q * N.kΩ R CM 6 V IN *Q optional. Provides predictable output current if any one RTD connection is broken: FIGUR. Three-Wire Connection for Remotely Located RTDs. Open RTD Terminal ma.6ma.6ma

9 There are special zener diode types specifically designed to provide a very low impedance clamp and withstand large energy surges. These devices normally have a diode characteristic in the forward direction which also protects against reversed loop connections. As noted earlier, reversed loop connections would produce a large loop current, possibly damaging R L. RADIO FRQUNCY INTRFRNC 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 RTD 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 often reduce or eliminate this 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. N D Use either D or D. See Reverse Voltage Protection. B D N R L V PS FIGUR. Reverse Voltage Protection. NOT: () Zener diode 6V: NA or General Semiconductor Transorb N66A Use lower voltage zener diodes with loop power supply voltages less than V for increased protection. B () R L V PS Maximum V PS must be less than minimum voltage rating of zener diode. FIGUR 6. Over-Voltage Surge Protection.

10 R Z V + IN RTD B 6 V IN R CM RLIN FIGUR. Input Bypassing Techniques. I R V + IN R G B Ω R 6 G V Pt IN R C to Z 6 C Ω.kΩ Ω N = -ma +V µf 6 RCV V O = to V V µf FIGUR. ±V-Powered Transmitter/Receiver Loop. RTD R Z I R V + IN B R 6 G V IN.kΩ N = -ma 6 RCV µf µf ISO 6 +V Isolated Power from PWS V V O V V FIGUR. Isolated Transmitter/Receiver Loop.

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