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Serial Input 16-Bit
4–20 mA, 0–20 mA DAC
4–20 mA, 0–20 mA or 0–24 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 VCC – 2.5 V
24-Pin SOIC and PDIP Packages
The AD420 is a complete digital to current loop output con-
verter, designed to meet the needs of the industrial control mar-
ket. It provides a high precision, fully integrated, low cost
single-chip solution for generating current loop signals in a com-
pact 24-pin 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
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
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
required (in addition to normal transient protection circuitry)
are three low cost capacitors which are used in the DAC output
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.
FUNCTIONAL BLOCK DIAGRAM
The AD420 is available in a 24-pin SOIC and PDIP over the in-
dustrial temperature range of –40°C to +85°C.
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
2. The AD420 is specified with a power supply range
from 12 V to 32 V. Output loop compliance is 0 V to
VCC – 2.5 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
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.
*SPI is a registered trademark of Motorola.
†MlCROWIRE is a registered trademark of National Semiconductor.
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.
© Analog Devices, Inc., 1995
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
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