a FEATURES 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
Serial Input 16-Bit 4–20 mA, 0–20 mA DAC AD420 FUNCTIONAL BLOCK DIAGRAM VCC VLL
AD420 REF IN DATA OUT CLEAR LATCH CLOCK DATA IN RANGE SELECT 1 RANGE SELECT 2
CLOCK DATA I/P REGISTER
IOUT 16-BIT DAC
SWITCHED CURRENT SOURCES AND FILTERING
1.25kΩ FAULT DETECT
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-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 separate 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 commonly 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 required (in addition to normal transient protection circuitry) are three 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.
The AD420 is available in a 24-pin 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 VCC – 2.5 V. 3. The flexible serial input can be used in three-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 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 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. *SPI is a registered trademark of Motorola. †MlCROWIRE is a registered trademark of National Semiconductor.
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