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PDF AD694 Data sheet ( Hoja de datos )
| Número de pieza | AD694 | |
| Descripción | 4.20 mA Transmitter | |
| Fabricantes | Analog Devices | |
| Logotipo |  |
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FEATURES
4–20 mA, 0–20 mA Output Ranges
Precalibrated Input Ranges:
0 V to 2 V, 0 V to 10 V
Precision Voltage Reference
Programmable to 2.000 V or 10.000 V
Single or Dual Supply Operation
Wide Power Supply Range: 4.5 V to 36 V
Wide Output Compliance
Input Buffer Amplifier
Open-Loop Alarm
Optional External Pass Transistor to Reduce
Self-Heating Errors
0.002% Typ Nonlinearity
4–20 mA Transmitter
AD694
FUNCTIONAL BLOCK DIAGRAM
PRODUCT DESCRIPTION
The AD694 is a monolithic current transmitter that accepts
high level signal inputs to drive a standard 4–20 mA current
loop for the control of valves, actuators, and other devices com-
monly used in process control. The input signal is buffered by
an input amplifier that can be used to scale the input signal or
buffer the output from a current mode DAC. Precalibrated in-
put spans of 0 V to 2 V and 0 V to 10 V are selected by simple
pin strapping; other spans may be programmed with external
resistors.
The output stage compliance extends to within 2 V of VS and
its special design allows the output voltage to extend below
common in dual supply operation. An alarm warns of an open
4–20 mA loop or noncompliance of the output stage.
Active laser trimming of the AD694’s thin film resistors results
in high levels of accuracy without the need for additional adjust-
ments and calibration. An external pass transistor may be used
with the AD694 to off-load power dissipation, extending the
temperature range of operation.
The AD694 is the ideal building block for systems requiring
noise immune 4–20 mA signal transmission to operate valves,
actuators, and other control devices, as well as for the transmis-
sion of process parameters such as pressure, temperature, or
flow. It is recommended as a replacement for discrete designs in
a variety of applications in industrial process control, factory
automation, and system monitoring.
The AD694 is available in hermetically sealed, 16-pin CERDIP
and plastic SOIC, specified over the –40°C to +85°C industrial
temperature range, and in a 16-pin plastic DIP, specified over
the 0°C to +70°C temperature range.
PRODUCT HIGHLIGHTS
1. The AD694 is a complete voltage in to 4–20 mA out current
transmitter.
2. Pin programmable input ranges are precalibrated at 0 V to
2 V and 0 V to 10 V.
3. The input amplifier may be configured to buffer and scale the
input voltage, or to serve as an output amplifier for current
output DACs.
4. The output voltage compliance extends to within 2 V of the
positive supply and below common. When operated with a
5 V supply, the output voltage compliance extends 30 V be-
low common.
5. The AD694 interfaces directly to 8-, 10-, and 12-bit single
supply CMOS and bipolar DACs.
6. The 4 mA zero current may be switched on and off with a
TTL control pin, allowing 0–20 mA operation.
7. An open collector alarm warns of loop failure due to open
wires or noncompliance of the output stage.
8. A monitored output is provided to drive an external pass
transistor. The feature off-loads power dissipation to extend
the temperature range of operation and minimize self-heating
error.
REV. B
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 that
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: 781/329-4700
www.analog.com
Fax: 781/326-8703
© Analog Devices, Inc., 2002
1 page  
AD694
optional NPN pass transistor can be added to transfer most of
the power dissipation off-chip, to extend the temperature range
of operation.
The output stage is current-limited at approximately 38 mA to
protect the output from an overdrive at its inputs. The V/I will
allow linear operation to approximately 24 mA. The V/I con-
verter also has an open collector alarm (Pin 10) which warns of
open-circuit condition at the IOUT pin or of attempts to drive the
output to a voltage greater than VS – 2 V.
4 mA OFFSET GENERATOR
This circuit converts a constant voltage from the voltage
reference to a constant current of approximately 200 µA. This
current is summed with the signal current at Pin 14 (BW
Adjust), to result in a constant 4 mA offset current at IOUT. The
4 mA Adj (Pin 6) allows the offset current to be adjusted to any
current in the range of 2 mA to 4.8 mA. Pin 9 (4 mA On/Off)
can shut off the offset current completely if it is lifted to 3.0 V or
more, allowing 0 to 20 mA operation of the AD694. In normal
4–20 mA operation, Pin 9 is connected to ground.
VOLTAGE REFERENCE
A 2 V or 10 V voltage reference is available for user applications,
selectable by pin-strapping. The 10 V option is available for
supply voltages greater than 12.5 V, the 2 V output is available
over the whole 4.5 V to 36 V power supply range. The reference
can source up to 5 mA for user applications. A boost transistor
can be added to increase the current drive capability of the 2 V
mode.
APPLYING THE AD694
The AD694 can easily be connected for either dual or single
supply operation, to operate from supplies as low as 4.5 V and
as high as 36 V. The following sections describe the different
connection configurations, as well as adjustment methods.
Table I shows possible connection options.
Table I. Precalibrated Ranges for the AD694
Input Output Voltage Min
Range Range Reference VS
Pin 9 Pin 4 Pin 8
0–2 V
0–10 V
0–2.5 V
0–12.5 V
0–2 V
0–10 V
0–2.5 V
0–12.5 V
4–20 mA
4–20 mA
0–20 mA
0–20 mA
4–20 mA
4–20 mA
0–20 mA
0–20 mA
2V
2V
2V
2V
10 V
10 V
10 V
10 V
4.5 V
12.5 V
5.0 V
15.0 V
12.5 V
12.5 V
12.5 V
15.0 V
Pin 5
Pin 5
≥3 V
≥3 V
Pin 5
Pin 5
≥3 V
≥3 V
Pin 5
Open
Pin 5
Open
Pin 5
Open
Pin 5
Open
Pin 7
Pin 7
Pin 7
Pin 7
Open
Open
Open
Open
BASIC CONNECTIONS: 12.5 V SINGLE-SUPPLY
OPERATION WITH 10 V FS
Figure 2 shows the minimal connections required for basic
operation with a 12.5 V power supply, 10 V input span,
4–20 mA output span, and a 10 V voltage reference. The buffer
amplifier is connected as a voltage follower to drive the V/I
converter by connecting FB (Pin 1) to –Sig (Pin 2). 4 mA On/
Off (Pin 9) is tied to ground (Pin 5) to enable the 4 mA offset
current. The AD694 can drive a maximum load RL = [VS – 2 V]
/20 mA, thus the maximum load with a 12.5 V supply is 525 Ω.
SELECTING A 2 V FULL-SCALE INPUT
The 2 V full-scale option is selected by shorting Pin 4 (2 V FS)
to Pin 5 (Common). The connection should be as short as pos-
sible; any parasitic resistance will affect the precalibrated span
accuracy.
SELECTING THE 2 V VOLTAGE REFERENCE
The voltage reference is set to a 2 V output by shorting Pin 7 to
Pin 8 (10 V Force to 2 V Sense). If desired, the 2 V reference
can be set up for remote force and sense connection. Keep in
mind that the 2 V Sense line carries a constant current of 100 µA
that could cause an offset error over long wire runs. The 2 V
reference option can be used with all supply voltages greater
than 4.5 V.
Figure 2. Minimal Connections for 0 V to 10 V Single-Ended Input, 4–20 mA Output, 10 V Reference Output
REV. B
–5–
5 Page 
AD694
Figure 13. Digital to 4–20 mA Interface Using a Current Steering DAC
Figure 14. Single-Supply Digital Input to 4–20 mA Output
code dependent, and the response time of the circuit will be de- some resistors, an instrumentation amplifier front end can be
termined by the reaction of the voltage reference. The supply
added which easily handles these types of low level signals.
voltage to the AD7541A should be kept close to 15 V. If VS is
reduced significantly from 15 V the differential nonlinearity of
the DAC will increase and the linearity will be degraded.
The traditional 3 op amp instrumentation amplifier is built us-
ing an AD708 dual op amp for the front end, and the AD694’s
buffer amplifier is used for the subtractor circuit, as shown in
In some applications it is desirable to have some underrange and Figure 15. The AD694’s 2 V reference is used to provide a
overrange in the 4–20 mA output. For example, assume an over “ground” of 2 V that ensures proper operation of the in amp
and under range capability of ± 5% of span is needed, then the
over a wide common mode range. The reference pin of the
output current range corresponding to the full scale of the DAC subtractor circuit is tied to the 2 V reference (point C). A 2 kΩ
is 3.2 mA to 20.8 mA. To accomplish this, the span of the
pull-down resistor ensures that the voltage reference will be able
AD694 would be increased 10% to 17.6 mA by adding a nonin- to sink any subtractor current. The 2 V FS (Pin 4) is attached to
verting gain of 1.1 to the buffer amplifier. The 4 mA offset
the 2 V reference; this offsets the input range of the V/I con-
would then be reduced by 0.8 mA, by utilizing the adjustment
verter 2 volts positive, to match the “ground” of the in amp.
scheme explained in Adjusting 4 mA Zero section. Then a digi-
tal input from all zero code to full scale would result in an out-
put current of 3.2 mA to 20.8 mA.
The AD694 will now output a 4–20 mA output current for a
0 V to 2 V differential swing across VA. The gain of the in amp
front end is adjusted so that the desired full-scale input signal at
LOW COST SENSOR TRANSMITTER
Sensor bridges typically output differential signals in the 10 mV
to 100 mV full-scale range. With an AD694, a dual op amp, and
VIN results in a VA of 2 V. For example a sensor that has a 100
mV full scale will require a gain of 20 in the front end. The gain
is determined according to the equation:
G = [2RS/Rg] + 1
REV. B
–11–
11 Page | ||
| Páginas | Total 16 Páginas | |
| PDF Descargar | [ Datasheet AD694.PDF ] | |
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