a
FEATURES
4 mA–20 mA, 0 mA–20 mA or 0 mA–24 mA Current Output
16-Bit Resolution and Monotonicity
0.012% Max Integral Nonlinearity
0.05% Max Offset (Trimmable)
0.15% Max Total Output Error (Trimmable)
Flexible Serial Digital Interface (3.3 MBPS)
On-Chip Loop Fault Detection
On-Chip 5 V Reference (25 ppm/ C Max)
Asynchronous CLEAR Function
Maximum Power Supply Range of 32 V
Output Loop Compliance of 0 V to V
CC
– 2.5 V
24-Lead SOIC and PDIP Packages
PRODUCT DESCRIPTION
Serial Input 16-Bit
4 mA–20 mA, 0 mA–20 mA DAC
AD420
FUNCTIONAL BLOCK DIAGRAM
V
CC
V
LL
REF OUT
REFERENCE
4k
40
BOOST
AD420
REF IN
DATA OUT
CLEAR
LATCH
CLOCK
DATA IN
RANGE
SELECT 1
RANGE
SELECT 2
CLOCK
DATA I/P
REGISTER
I
OUT
16-BIT
DAC
SWITCHED
CURRENT
SOURCES
AND
FILTERING
V
OUT
1.25k
FAULT
DETECT
The AD420 is a complete digital to current loop output con-
verter, designed to meet the needs of the industrial control
market. It provides a high precision, fully integrated, low cost
single-chip solution for generating current loop signals in a
compact 24-lead SOIC or PDIP package.
The output current range can be programmed to 4 mA–20 mA,
0 mA–20 mA or an overrange function of 0 mA–24 mA. The
AD420 can alternatively provide a voltage output from a sepa-
rate pin that can be configured to provide 0 V–5 V, 0 V–10 V,
±
5 V or
±
10 V with the addition of a single external buffer
amplifier.
The 3.3M Baud serial input logic design minimizes the cost of
galvanic isolation and allows for simple connection to com-
monly used microprocessors. It can be used in three-wire or
asynchronous mode and a serial-out pin is provided to allow
daisy chaining of multiple DACs on the current loop side of the
isolation barrier.
The AD420 uses sigma-delta (Σ∆) DAC technology to achieve
16-bit monotonicity at very low cost. Full-scale settling to 0.1%
occurs within 3 ms. The only external components that are re-
quired (in addition to normal transient protection circuitry) are
two low cost capacitors which are used in the DAC output filter.
If the AD420 is going to be used at extreme temperatures and
supply voltages, an external output transistor can be used to
minimize power dissipation on the chip via the “BOOST” pin.
The FAULT DETECT pin signals when an open circuit occurs
in the loop. The on-chip voltage reference can be used to supply
a precision +5 V to external components in addition to the
AD420 or, if the user desires temperature stability exceeding
25 ppm/°C, an external precision reference such as the AD586
can be used as the reference.
SPI is a registered trademark of Motorola.
MICROWIRE is a registered trademark of National Semiconductor.
OFFSET CAP 1
TRIM
CAP 2
GND
The AD420 is available in a 24-lead SOIC and PDIP over the
industrial temperature range of –40°C to +85°C.
PRODUCT HIGHLIGHTS
1. The AD420 is a single chip solution for generating 4 mA–
20 mA or 0 mA–20 mA signals at the “controller end” of the
current loop.
2. The AD420 is specified with a power supply range
from 12 V to 32 V. Output loop compliance is 0 V to
V
CC
– 2.5 V.
3. The flexible serial input can be used in three-wire mode
with SPI
®
or MICROWIRE
®
microcontrollers, or in asyn-
chronous mode which minimizes the number of control
signals required.
4. The serial data out pin can be used to daisy chain any num-
ber of AD420s together in three-wire mode.
5. At power-up the AD420 initializes its output to the low end
of the selected range.
6. The AD420 has an asynchronous CLEAR pin which sends
the output to the low end of the selected range (0 mA,
4 mA, or 0 V).
7. The AD420 BOOST pin accommodates an external transis-
tor to off-load power dissipation from the chip.
8. The offset of
±
0.05% and total output error of
±
0.15% can
be trimmed if desired, using two external potentiometers.
REV. F
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: 781/329-4700
World Wide Web Site: http://www.analog.com
Fax: 781/326-8703
© Analog Devices, Inc., 1999
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