Mike Caruso, Sensor Applications Engineer AN-102 This applic

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Mike Caruso, Sensor Applications Engineer AN-102 This applic

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4-20mA CURRENT LOOP PPT/PPT-R
Mike Caruso, Sensor Applications Engineer
AN-102
This application note describes 4-20 current loop interface circuit Precision Pressure Transducer (PPT) family. This family pertains both PPT-R type devices. family offer high accuracy smart pressure sensing with network capability affordable price. Pressure output available RS-232 RS-485) digital value well volt analog voltage. benefit using analog output that there need offset span adjustments achieve accuracy 0.12% full scale (FS) typical across (-40 temperature range. Even though there capability user adjust store null span correction, stability less than 0.025% drift/year minimizes calibration concerns. Every unit both digital analog outputs individually calibrated factory temperature variations throughout range. 4-20 current loop output standard interface many industrial process control factory automation systems. current loop output provides noise immunity long distances factory floor environment. many existing systems, easier retrofit pressure sensors 4-20 loop than replace whole system configuration. This application note allows advanced smart pressure sensors (PPT PPT-R) replace conventional pressure sensors wire 4-20 current loop. single capacitor that
required job. Analog Devices AD694 provides circuitry control necessary drive current loop. provides configuration necessary scale output range fine adjust offset span values. Figure shows circuit connections.
4-20mA CURRENT LOOP EXAMPLE
Start with 20psig with null (00) address. desired configuration output inches water column (1.5psi) 20mA output inches water column (15psi). Integrate pressure readings over second period reduce noise. Connect computer type following:
*00WE=RAM *00DU=INWC *00DA=B *00F=400 *00W=90 *00O=10 *00H=40 *00AN=ON *00I=M10 *00WE *00SP=ALL Enable accept commands units inches water column (inwc) Connect pressure analog full scale (FS) inwc press. window 90%FS (360 inwc) press. offset 10%FS inwc) output high level volts Output range 40-400 inwc, volts integration time sec. Store settings EEPROM
Connect shown Figure When power applied, will output pressure from inwc inwc 20mA.
Vsupply=5.5-30V
-Sig +Sig Force Sense Iout On/Off
Iout=4-20
Pressure Input
Analog
AD694 4-20mA Transmitter
Honeywell
Ground
Ground
(max) (Vsupply 2V)/20mA
Figure Four wire interface 4-20 current loop
4-20mA CURRENT LOOP PPT/PPT-R
fine adjustments offset span necessary, reconnect computer commands adjust gain offset parameters. First, adjust offset parameter (Z=) with inwc applied. Then adjust gain parameter (X=) with inwc applied.
ALARM CONDITION
AD694 built alarms that that warn open circuit Iout (pin attempt drive voltage Iout higher than Vsupply-2V. alarm output (pin open collector output that drives logic level (typically 0.35V) alarm conditions. drive current alarm limited about 20mA.
output, Analog will respond computer command inputs using command. Since analog output range from zero volts, commands N=2000 will drive loop currents from analog output level controlled tenth millivolt entering: drive 1.2561 volts Analog PPT. This correspond 14.049mA. relation ship between command value Iout current expressed Iout (mA) [(N=/2000) this mode, analog output voltage fine adjusted using commands endpoint trimming levels.
TESTING 4-20mA LOOP
configured drive specific current levels 4-20mA loop (see Figure This useful test loop connection check system interface circuitry. this mode, must configured allow host computer access drive analog output signal. This done setting Digital Analog Control command DA=G, command sequence below will configure this mode.
*00WE=RAM *00DA=R *00H=40 *00AN=ON *00NE=DAC *00WE *00SP=ALL Enable accept commands Allow values drive output output high level volts Output range volts Enable write signal Store settings EEPROM
Reference: Manual command descriptions
Customer Service Representative
612-954-2888 fax: 612-954-2582 E-Mail: clr@mn14.ssec.honeywell.com Site: www.ssec.honeywell.com
Analog
Vsupply=5.5-30V
-Sig +Sig Force Sense Iout On/Off
Iout=4-20
AD694 4-20mA Transmitter
*00N=0 (4mA) *00N=1000 (12mA) *00N=2000 (20mA)
Honeywell
Ground
Ground
Figure Computer control 4-20 current loop
10/96
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Mike Caruso, Sensor Applications Engineer AN-102 This applic

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