AD693 Loop-Powered 4-20 mA Sensor Transmitter

1 FUNCTIONAL BLOCK DIAGRAMREV. AInformation furnished by Analog Devices is believed to be accurate andreliable. However, no responsibility is assumed by Analog Devices for itsuse, nor for any infringements of patents or other rights of third partieswhich may result from its use. No license is granted by implication orotherwise under any patent or patent rights of Analog 4 20 mASensor TransmitterOne Technology Way, Box 9106, Norwood, MA 02062-9106, : 617/329-4700 Fax: 617/326-8703AD693 FEATURESI nstrumentation Amplifier Front EndLoop- powered OperationPrecalibrated 30 mV or 60 mV Input SpansIndependently Adjustable Output Span and ZeroPrecalibrated Output Spans: 4 20 mA Unipolar0 20 mA Unipolar12 6 8 mA BipolarPrecalibrated 100V RTD V Reference with Up to mA of Current AvailableUncommitted Auxiliary Amp for Extra FlexibilityOptional External Pass Transistor to ReduceSelf-Heating ErrorsPRODUCT DESCRIPTIONThe AD693 is a monolithic signal conditioning circuit whichaccepts low-level inputs from a variety of transducers to control astandard 4 20 mA, two-wire current loop.

2 An on-chip voltagereference and auxiliary amplifier are provided for transducerexcitation; up to mA of excitation current is available when thedevice is operated in the Loop-Powered mode. Alternatively, thedevice may be locally powered for three-wire applications when0 20 mA operation is 30 mV and 60 mV input spans may be set bysimple pin strapping. Other spans from 1 mV to 100 mV maybe realized with the addition of external resistors. The auxiliaryamplifier may be used in combination with on-chip voltages toprovide six precalibrated ranges for 100 RTDs. Output spanand zero are also determined by pin strapping to obtain thestandard ranges: 4 20mA, 12 8 mA and 0 20 laser trimming of the AD693 s thin-film resistors resultin high levels of accuracy without the need for additionaladjustments and calibration. Total unadjusted error is tested onevery device to be less than of full scale at +25 C, and lessthan over the industrial temperature range.

3 Residualnonlinearity is under The AD693 also allows for the useof an external pass transistor to further reduce errors caused transmission of low-level signals from RTDs, bridges andpressure transducers, the AD693 offers a cost-effective signalconditioning solution. It is recommended as a replacement fordiscrete designs in a variety of applications in process control,factory automation and system AD693 is packaged in a 20-pin ceramic side-brazed DIP,20-pin Cerdip, and 20-pin LCCC and is specified over the 40 C to +85 C industrial temperature HIGHLIGHTS1. The AD693 is a complete monolithic low-level voltage-to-current loop signal Precalibrated output zero and span options include4 20 mA, 0 20 mA, and 12 8 mA in two- and Simple resistor programming adds a continuum of rangesto the basic 30 mV and 60 mV input The common-mode range of the signal amplifier inputextends from ground to near the device s operating Provision for transducer excitation includes a Vreference output and an auxiliary amplifier which may beconfigured for voltage or current output and The circuit configuration permits simple linearization ofbridge, RTD, and other transducer A monitored output is provided to drive an external passtransistor.

4 This feature off-loads power dissipation toextend the temperature range of operation, enhancereliability, and minimize self-heating Laser-wafer trimming results in low unadjusted errors andaffords precalibrated input and output Zero and span are independently adjustable and noninteractiveto accommodate transducers or user defined Six precalibrated temperature ranges are available with a100 RTD via pin SPECIFICATIONSREV. A 2 (@ +258C and VS = +24 V. Input Span = 30 mV or 60 mV. Output Span = 4 20 mA,RL = 250 V, VCM = V, with external pass transistor unless otherwise noted.)ModelAD693AD/AQ/AEConditionsMinTy pMaxUnitsLOOP- powered OPERATIONTOTAL UNADJUSTED ERROR1, 2 Full ScaleTMIN to TMAX Full Scale100 RTD CALIBRATION ERROR3(See Figure 17) CLOOP powered OPERATION2 Zero Current Error4 Zero = 4 mA 25680 AZero = 12 mA 406120 AZero = 0 mA5+7+35+100 Avs. = 4 mA A/ CPower Supply Rejection (RTI)12 V VOP 36V6 V/V0 V VCM VCommon-Mode Input Range(See Figure 3)0+VOP 4 V6 VCommon-Mode Rejection (RTI)0 V VCM V 10630 V/VInput Bias Current7+5+20nATMIN to TMAX+7+25nAInput Offset Current7 VSIG = 0 mV Input mV Input Error Common-Mode0 V VCM V30 mV Input Span mV Input Span vs.

5 Temp. 20 50ppm/ CNonlinearity830 mV Input Span of Span60 mV Input Span of SpanOPERATIONAL VOLTAGE RANGEO perational Voltage, VOP6+12+36 VQuiescent CurrentInto Pin 9+500+700 AOUTPUT CURRENT LIMIT+21+25+32mACOMPONENTS OF ERRORSIGNAL AMPLIFIER9 Input Voltage Offset 406200 Vvs. Temp V/ CPower Supply Rejection12 V VOP 36 V6 V/V0 V VCM VV/I CONVERTER9, 10 Zero Current ErrorOutput Span = 4 20 mA 30 80 APower Supply Rejection12 V VOP 36 V6 Error V REFERENCE9, 12 Output Voltage Tolerance 3612mVvs. Temp. 20 50ppm/ CLine Regulation12 V VOP 36 V6 2006300 V/VLoad Regulation110 mA IREF 3 mA Current13 Loop powered , (Figure 10)+ + Mode, (Figure 15)+ AMPLIFIERC ommon-Mode Range0+VOP 4 V6 VInput Offset Voltage 50 200 VInput Bias Current+5+20nAInput Offset Current+ Rejection90dBPower Supply Rejection105dBOutput Current RangePin IX OUT+ +5mAOutput Current ErrorPin VX Pin IX RANGECase Operating14 TMIN to TMAX 40+85 CStorage 65+150 CNOTES1 Total error can be significantly reduced (typically less than ) by trimming the zero current.

6 The remaining unadjusted error sources are transconductance AD693 is tested as a loop powered device with the signal amp, V/I converter, voltage reference, and application voltages operating together. Specifications arevalid for preset spans and spans between 30 mV and 60 from ideal output assuming a perfect 100 RTD at 0 and +100 to the Error Analysis to calculate zero current error for input spans less than 30 forcing the differential signal amplifier input sufficiently negative the 7 A zero current can always be operational voltage (VOP) is the voltage directly across the AD693 (Pin 10 to 6 in two-wire mode, Pin 9 to 6 in local power mode). For example, VOP = VS (ILOOP RL) in two-wire mode (refer to Figure 10).7 Bias currents are not symmetrical with input signal level and flow out of the input pins. The input bias current of the inverting input increases with input signal volt-age, see Figure is defined as the deviation of the output from a straight line connecting the endpoints as the input is swept over a 30 mV and 60 mV input for the individual functional blocks are components of error that contribute to, and that are included in, the Loop powered Operation error contributions of V/I converter and Application in the reference output voltage due to load will affect the Zero Current.

7 A 1% change in the voltage reference output will result in an error of 1% in thevalue of the Zero not used for external excitation, the reference should be loaded by approximately 1 mA ( k to common).13In the loop powered mode up to 5 mA can be drawn from the reference, however, the lower limit of the output span will be increased accordingly. mA is themaximum current the reference can source while still maintaining a 4 mA AD693 is tested with a pass transistor so TA subject to change without shown in boldface are tested on all production units at final electrical test. Results from those tests are used to calculate outgoing quality levels. All minand max specifications are guaranteed, although only those shown in boldface are tested on all production MAXIMUM RATINGSS upply Voltage .. +36 VReverse Loop Current .. 200 mASignal Amp Input Range .. V to VOPR eference Short Circuit to Common .. IndefiniteAuxiliary Amp Input Voltage Range.

8 V to VOPA uxiliary Amp Current Output .. 10 mAStorage Temperature .. 65 C to +150 CLead Temperature, 10 sec Soldering .. +300 CMax Junction Temperature .. +150 CORDERING GUIDEP ackagePackageModelDescriptionOptionAD693 ADCeramic Side-Brazed DIPD-20AD693 AQCerdipQ-20AD693 AELeadless Ceramic ChipE-20 ACarrier (LCCC) AD693 PIN CONFIGURATION(AD, AQ, AE Packages)Functional Diagram 3 REV. AAD693 Figure 1. Maximum Load Resistancevs. Power SupplyFigure 2. Differential Input Current Signal Voltage Normalized to +INFigure 3. Maximum Common-ModeVoltage vs. SupplyAD693 Typical CharacteristicsREV. A 4 Figure 4. Bandwidth vs. Series LoadResistanceFigure 5. Signal Amplifier PSRR 6. CMRR (RTI) vs. FrequencyFigure 7. Input Current Noise 8. Input Voltage Noise A 5 converter s inverting input (Pin 12). Arranging the zero offset inthis way makes the zero signal output current independent ofinput span. When the input to the signal amp is zero, thenoninverting input of the V/I is at the standard offsets are laser trimmed at the factory,adjustment is seldom necessary except to accommodate the zerooffset of the actual source.

9 (See Adjusting Zero. )SIGNAL AMPLIFIERThe Signal Amplifier is an instrumentation amplifier used tobuffer and scale the input to match the desired span. Inputsapplied to the Signal Amplifier (at Pins 17 and 18) are amplifiedand referred to the V reference output in much the same way asthe level translation occurs in the V/I converter. Signals from thetwo preamplifiers are subtracted, the difference is amplified, andthe result is fed back to the upper preamp to minimize thedifference. Since the two preamps are identical, this minimum willoccur when the voltage at the upper preamp just matches thedifferential input applied to the Signal Amplifier at the the signal which is applied to the V/I is attenuated acrossthe two 800 resistors before driving the upper preamp, it willnecessarily be an amplified version of the signal applied betweenPins 17 and 18. By changing this attenuation, you can controlthe span referred to the Signal Amplifier. To illustrate: a 75 mVsignal applied to the V/I results in a 20 mA loop , 15 mV is applied to offset the zero to 4 mA leaving a60 mV range to correspond to the span.

10 And, since the nominalattenuation of the resistors connected to Pins 16, 15 and 14 , a 30 mV input signal will be doubled to result in 20 mA ofloop current. Shorting Pins 15 and 16 results in unity gain andpermits a 60 mV input span. Other choices of span may beimplemented with user supplied resistors to modify theattenuation. (See section Adjusting Input Span. )The Signal Amplifier is specially designed to accommodate alarge common-mode range. Common-mode signals anywhere upto and beyond the V reference are easily handled as long asVIN is sufficiently positive. The Signal Amplifier is biased withrespect to VIN and requires about volts of headroom. Theextended range will be useful when measuring sensors driven,for example, by the auxiliary amplifier which may go above V potential. In addition, the PNP input stage will continueto operate normally with common-mode voltages of severalhundred mV, negative, with respect to common. This featureaccommodates self-generating sensors, such as thermocouples,which may produce small negative normal-mode signals as wellas common-mode noise on grounded signal AMPLIFIERThe Auxiliary Amplifier is included in the AD693 as a signalconditioning aid.