Loop-Powered
4 mA to 20 mA DAC
a
AD421
FEATURES
FUNCTIONAL BLOCK DIAGRAM
4 mA to 20 mA Current Output
HART® Compatible
REF IN
(+2.5V)
REF OUT2
(+2.5V)
REF OUT1
(+1.25V)
16-Bit Resolution and Monotonicity
؎0.01% Integral Nonlinearity
5 V or 3 V Regulator Output
2.5 V and 1.25 V Precision Reference
750 A Quiescent Current max
Programmable Alarm Current Capability
Flexible High Speed Serial Interface
16-Lead SOIC and PDIP Packages
LV
V
CC
75k⍀
112.5k⍀
AD421
134k⍀
DRIVE
COMP
BANDGAP
REFERENCE
121k⍀
BOOST
LOCAL
OSCILLATOR
DATA
INPUT SHIFT
REGISTER
CLOCK
SWITCHED
CURRENT
SOURCES
AND
16-BIT
SIGMA-
DELTA DAC
DAC LATCH
LATCH
40⍀
80k⍀
GENERAL DESCRIPTION
FILTERING
LOOP
RTN
POWER-ON
RESET
The AD421 is a complete, loop-powered, digital to 4 mA to
20 mA converter, designed to meet the needs of smart trans-
mitter manufacturers in the Industrial Control industry. It pro-
vides a high precision, fully integrated, low cost solution in a
compact 16-lead package. The AD421 is ideal for extending the
resolution of smart 4 mA to 20 mA transmitters at very low cost.
COM
C1 C2 C3
PRODUCT HIGHLIGHTS
1. The AD421 is a single chip, high performance, low cost
solution for generating 4 mA to 20 mA signals for smart
industrial control transmitters.
The AD421 includes a selectable regulator that is used to power
itself and other devices in the transmitter. This regulator pro-
vides either a +5 V, +3.3 V or +3 V regulated output voltage.
The part also contains +1.25 V and +2.5 V precision references.
The AD421 thus eliminates the need for a discrete regulator
and voltage reference. The only external components required
are a number of passive components and a pass transistor to
span large loop voltages.
2. The AD421’s regulated supply voltage can be used to power
any additional circuits in the transmitter. The regulated
output value is pin selectable as either +3 V, +3.3 V or +5 V.
3. The AD421’s on-chip references can provide a precision
reference voltage to other devices in the system. This refer-
ence voltage can be either +1.25 V or +2.5 V.
The AD421 can be used with standard HART FSK protocol
communication circuitry without any degradation in specified
performance. The high speed serial interface is capable of oper-
ating at 10 Mbps and allows for simple connection to com-
monly-used microprocessors and microcontrollers via a standard
three-wire serial interface.
4. The AD421 is fully compatible with standard HART cir-
cuitry or other similar FSK protocols.
5. With the addition of a single discrete transistor, the AD421
can be operated from VCC + 2 V min to a maximum of the
breakdown voltage of the pass transistor.
The sigma-delta architecture of the DAC guarantees 16-bit
monotonicity while the integral nonlinearity for the AD421 is
0.01%. The part provides a zero scale 4 mA output current
with 0.1% offset error and a 20 mA full-scale output current
with 0.2% gain error.
6. The AD421 converts the digital data to current with 16-bit
resolution and monotonicity. Full-scale settling time to
0.1% typically occurs within 8 ms.
7. The AD421 features a programmable alarm current capabil-
ity that allows the transmitter to send out of range currents to
indicate a transducer fault.
The AD421 is available in a 16-lead, 0.3 inch-wide, plastic DIP
and in a 16-lead, 0.3 inch-wide, SOIC package. The part is speci-
fied over the industrial temperature range of –40°C to +85°C.
HART is a registered trademark of the HART Communication
Foundation.
REV. C
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
Fax: 781/326-8703
World Wide Web Site: http://www.analog.com
© Analog Devices, Inc., 2000
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