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부품번호 AD694 기능
기능 4.20 mA Transmitter
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AD694 데이터시트, 핀배열, 회로
a
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




AD694 pdf, 반도체, 판매, 대치품
AD694
FUNCTIONAL DESCRIPTION
The operation of the AD694 can best be understood by dividing
the circuit into three functional parts (see Figure 1). First, a
single supply input amplifier buffers the high level, single-ended
input signal. The buffer amplifier drives the second section, a
voltage to current (V/I) converter, that makes a 0 to 16 mA sig-
nal dependent current.
Typical Minimum Supply Voltage vs. Temperature for 2 V
and 10 V Full Scale
Maximum RL vs. Supply Voltage
Voltage Reference Power Supply Rejection
IOUT: Voltage Compliance vs. Temperature
Figure 1. Functional Block Diagram
The third section, a voltage reference and offset generator, is re-
sponsible for providing the 4 mA offset current signal.
BUFFER AMPLIFIER
The buffer amplifier is a single supply amplifier that may be
used as a unity gain buffer, an output amplifier for a current
output DAC, or as a gain block to amplify low level signals. The
amplifier’s PNP input stage has a common-mode range that ex-
tends from a few hundred mV below ground to within 2.5 V of
VS. The Class A output of the amplifier appears at Pin 1 (FB).
The output range extends from about 1 mV above common to
within 2.5 V of VS when the amplifier is operated as a follower.
The amplifier can source a maximum load of 5 k, but can sink
only as much as its internal 10 kpulldown resistor allows.
V/I CONVERTER
The ground referenced, input signal from the buffer amplifier is
converted to a 0 to 0.8 mA current by A2 and level shifted to
the positive supply. A current mirror then multiplies this signal
by a factor of 20 to make the signal current of 0 to 16 mA. This
technique allows the output stage to drive a load to within 2 V
of the positive supply (VS). Amplifier A2 forces the voltage at
Pin 1 across resistors R1 and R2 by driving the Darlington tran-
sistor, Q2. The high gain Darlington transmits the resistor cur-
rent to its collector and to R3 (900 ). A3 forces the level
shifted signal across the 45 resistor to get a current gain of 20.
The transfer function of the V/I stage is therefore:
( )IOUT = 20 ×VPIN1 / R1 + R2
resulting in a 0-16 mA output swing for a 0–10 V input. Tying
Pin 4 (2 V FS) to ground shorts out R2 and results in a 2 V
full-scale input for a 16 mA output span.
The output stage of the V/I converter is of a unique design that
allows the IOUT pin to drive a load below the common (sub-
strate) potential of the device. The output transistor can always
drive a load to a point 36 V below the positive supply (VS). An
–4– REV. B

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AD694 전자부품, 판매, 대치품
AD694
Figure 4. Using Optional Pass Transistor to Minimize Self-Heating Errors; Dual Supply Operation Shown
POWER DISSIPATION CONSIDERATIONS
The AD694 is rated for operation over its specified temperature
without the use of an external pass transistor. However, it is
possible to exceed the absolute maximum power dissipation,
with some combinations of power supply voltage and voltage
reference load. The internal dissipation of the part can be calcu-
lated to determine if there is a chance that the absolute maxi-
mum dissipation may be exceeded. The die temperature must
never exceed 150°C.
Total power dissipation (PTOT), is the sum of power dissipated
by the internal amplifiers, P (Standing), the voltage reference,
P(VREF) and the current output stage, P(IOUT) as follows:
PTOT = P (Standing) + P (VREF) + P (IOUT)
where:
P (Standing) = 2 mA (max) × VS
P (VREF) = (VS – VREF) × IVREF
P(IOUT) (VS – VOUT) × IOUT (max):
IOUT (max) may be the max expected operating cur-
rent, or the overdriven current of the device.
P(IOUT) drops to (2 V × IOUT) if a pass transistor
is used.
Definitions:
VREF = output voltage of reference
IVREF = output current of reference
VS = supply voltage
VOUT = voltage at IOUT pin.
An appropriate safety factor should be added to PTOT.
The junction temperature may be calculated with the following
formula:
TJ = PTOT (θJC + θCA) + TAMBIENT
θJC is the thermal resistance between the chip and the package
(case), θCA is the thermal resistance between the case and its
surroundings and is determined by the characteristics of the
thermal connection of the case to ambient.
For example, assume that the part is operating with a VS of 24 V
in the CERDIP package at 50°C, with a 1 mA load on the 10 V
reference. Assume that IOUT is grounded and that the max IOUT
would be 20 mA. The internal dissipation would be:
P(TOT) = 2 mA × 24 V + (24 V – 10 V) × 1 mA + (24 V – 0 V) × 20 mA
= 48 mW + 14 mW + 480 mW = 542 mW
Using θJC of 30°C/W and θCA of 70°C/W (from specifications
page), the junction temperature is:
TJ = 542 mW (30°C/W + 70°C/W) + 50°C = 104.2°C
The junction temperature is in the safe region.
Internal power dissipation can be reduced either by reducing the
value of θCA through the use of air flow or heat sinks, or by re-
ducing PTOT of the AD694 through the use of an external pass
transistor. Figure 5 shows the maximum case and still air tem-
peratures for a given level of power dissipation.
Figure 5. Internal Power Dissipation in mW
ADJUSTMENT PROCEDURES
The following sections describe methods for trimming the out-
put current offset, the span, and the voltage reference.
ADJUSTING 4 mA ZERO
The 4 mA zero current may be adjusted over the range of 2 mA
to 4.8 mA to accommodate large input signal offsets, or to allow
small adjustment in the zero current. The zero may be adjusted
by pulling up or down on Pin 6 (4 mA Adj) to increase or de-
crease the nominal offset current. The 4 mA Adj. (Pin 6) should
not be driven to a voltage greater than 1 V. The arrangement of
REV. B
–7–

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