Serial Input 16-Bit
4 mA–20 mA, 0 mA–20 mA DAC
AD420
FUNCTIONAL BLOCK DIAGRAM
FEATURES
V
CC
4 mA–20 mA, 0 mA–20 mA or 0 mA–24 mA
current output
16-bit resolution and monotonicity
0.012ꢀ max integral nonlinearity
0.0ꢁꢀ max offset (trimmable)
V
REFERENCE
AD420
LL
4kΩ
40Ω
REF OUT
BOOST
REF IN
0.1ꢁꢀ max total output error (trimmable)
Flexible serial digital interface (3.3 MBPS)
On-Chip loop fault detection
On-chip ꢁ V reference (2ꢁ ppm/°C max)
Asynchronous CLEAR function
CLOCK
DATA OUT
CLEAR
LATCH
CLOCK
DATA IN
I
OUT
DATA I/P
REGISTER
V
OUT
SWITCHED
CURRENT
SOURCES
AND
16-BIT
DAC
1.25kΩ
FAULT
DETECT
RANGE
SELECT 1
FILTERING
RANGE
SELECT 2
Maximum power supply range of 32 V
Output loop compliance of 0 V to VCC − 2.7ꢁ V
24-Lead SOIC and PDIP packages
OFFSET CAP 1 CAP 2
TRIM
GND
Figure 1.
GENERAL DESCRIPTION
The AD420 is a complete digital to current loop output
converter, 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.
user desires temperature stability exceeding 25 ppm/°C, an
external precision reference such as the AD586 can be used as
the reference. The AD420 is available in a 24-lead SOIC and
PDIP over the industrial temperature range of −40°C to +85°C.
PRODUCT HIGHLIGHTS
The output current range can be programmed to 4 mA to
20 mA, 0 mA to 20 mA or to an overrange function of 0 mA to
24 mA. The AD420 can alternatively provide a voltage output from
a separate pin that can be configured to provide 0 V to 5 V, 0 V
to 10 V, 5 V, or 10 V with the addition of a single external
buffer amplifier.
1. The AD420 is a single chip solution for generating 4 mA to
20 mA or 0 mA to 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 VCC − 2.75 V.
3. The flexible serial input can be used in 3-wire mode
with SPI® or MICROWIRE® microcontrollers, or in
asynchronous mode, which minimizes the number of
control signals required.
The 3.3 M Baud serial input logic design minimizes the cost of
galvanic isolation and allows for simple connection to commonly
used microprocessors. It can be used in 3-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.
4. The serial data out pin can be used to daisy chain any
number of AD420s together in 3-wire mode.
The AD420 uses sigma-delta (Σ-Δ) 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
required (in addition to normal transient protection circuitry)
are two low cost capacitors which are used in the DAC out-
put filter.
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).
If the AD420 is 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
7. The AD420 BOOST pin accommodates an external
transistor 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. H
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. Specifications subject to change without notice. No
license is granted by implication or otherwise under any patent or patent rights of Analog Devices.
Trademarks and registeredtrademarks arethe property of their respective owners.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781.329.4700
www.analog.com
Fax: 781.461.3113 ©1999–2011 Analog Devices, Inc. All rights reserved.
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