Loop-Powered 4–20 mA
Sensor Transmitter
a
AD693
FUNCTIONAL BLOCK DIAGRAM
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
PRODUCT DESCRIPTION
PRODUCT HIGHLIGHTS
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 loop-powered 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-to-
current loop signal conditioner.
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.
4. The common-mode range of the signal amplifier input
extends from ground to near the device’s operating voltage.
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.
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.
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.
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.
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.
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.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 617/329-4700
Fax: 617/326-8703
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