a FEATURES Instrumentation Amplifier Front End Loop-Powered Operation Precalibrated 30 mV or 60 mV Input Spans Independently Adjustable Output Span and Zero Precalibrated Output Spans: 4–20 mA Unipolar 0–20 mA Unipolar 12 ؎ 8 mA Bipolar Precalibrated 100 ⍀ RTD Interface 6.2 V Reference with Up to 3.5 mA of Current Available Uncommitted Auxiliary Amp for Extra Flexibility Optional External Pass Transistor to Reduce Self-Heating Errors
Loop-Powered 4–20 mA Sensor Transmitter AD693 FUNCTIONAL BLOCK DIAGRAM
The AD693 is a monolithic signal conditioning circuit which accepts low-level inputs from a variety of transducers to control a standard 4–20 mA, two-wire current loop. An on-chip voltage reference and auxiliary amplifier are provided for transducer excitation; up to 3.5 mA of excitation current is available when the device is operated in the loopowered mode. Alternatively, the device may be locally powered for three-wire applications when 0–20 mA operation is desired.
1. The AD693 is a complete monolithic low-level voltage-tocurrent loop signal conditioner.
Precalibrated 30 mV and 60 mV input spans may be set by simple pin strapping. Other spans from 1 mV to 100 mV may be realized with the addition of external resistors. The auxiliary amplifier may be used in combination with on-chip voltages to provide six precalibrated ranges for 100 Ω RTDs. Output span and zero are also determined by pin strapping to obtain the standard ranges: 4–20mA, 12 ± 8 mA and 0–20 mA.
4. The common-mode range of the signal amplifier input extends from ground to near the device’s operating voltage.
Active laser trimming of the AD693’s thin-film resistors result in high levels of accuracy without the need for additional adjustments and calibration. Total unadjusted error is tested on every device to be less than 0.5% of full scale at +25°C, and less than 0.75% over the industrial temperature range. Residual nonlinearity is under 0.05%. The AD693 also allows for the use of an external pass transistor to further reduce errors caused by self-heating. For transmission of low-level signals from RTDs, bridges and pressure transducers, the AD693 offers a cost-effective signal conditioning solution. It is recommended as a replacement for discrete designs in a variety of applications in process control, factory automation and system monitoring.
2. Precalibrated output zero and span options include 4–20 mA, 0–20 mA, and 12 ± 8 mA in two- and three-wire configurations. 3. Simple resistor programming adds a continuum of ranges to the basic 30 mV and 60 mV input spans.
5. Provision for transducer excitation includes a 6.2 V reference output and an auxiliary amplifier which may be configured for voltage or current output and signal amplification. 6. The circuit configuration permits simple linearization of bridge, RTD, and other transducer signals. 7. A monitored output is provided to drive an external pass transistor. This feature off-loads power dissipation to extend the temperature range of operation, enhance reliability, and minimize self-heating errors. 8. Laser-wafer trimming results in low unadjusted errors and affords precalibrated input and output spans. 9. Zero and span are independently adjustable and noninteractive to accommodate transducers or user defined ranges. 10. Six precalibrated temperature ranges are available with a 100 Ω RTD via pin strapping.
The 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 range. REV. A Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices.
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