Serial Input 16-Bit
4 mA–20 mA, 0 mA–20 mA DAC
Data Sheet
AD420
Rev. I Document Feedback
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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
− 2.75 V
24-Lead SOIC and PDIP packages
FUNCTIONAL BLOCK DIAGRAM
FAULT
DETECT
I
OUT
BOOST
40Ω
1.25kΩ
4kΩ
V
OUT
OFFSET
TRIM
CAP 1 CAP 2 GND
V
LL
CC
REF OUT
REF IN
DATA OUT
CLEAR
LATCH
CLOCK
DATA IN
RANGE
SELECT 1
RANGE
SELECT 2
AD420
REFERENCE
DATA I/P
REGISTER
SWITCHED
CURRENT
SOURCES
AND
FILTERING
CLOCK
16-BIT
DAC
00494-001
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.
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.
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.
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.
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
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
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 V
CC
− 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.
4. The serial data out pin can be used to daisy chain any
number of AD420s together in 3-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
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.
