AD5751 datasheet(21/22 Pages) AD

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Preliminary Technical Data

AD5751

Rev. PrA | Page 21 of 22

APPLICATIONS INFORMATION

TRANSIENT VOLTAGE PROTECTION

The AD5751 contains ESD protection diodes which prevent

damage from normal handling. The industrial control

environment can, however, subject I/O circuits to much higher

transients. In order to protect the AD5751 from excessively high

voltage transients , external power diodes and a surge current

limiting resistor may be required, as shown in Figure 15. The

constraint on the resistor value is that during normal operation

the output level at IOUT must remain within its voltage

compliance limit of AVDD – 2.0V and the two protection diodes

and resistor must have appropriate power ratings.

IOUT

AVDD

RLOAD

AVDD

AD5751

Figure 15. Output Transient Voltage Protection

LAYOUT GUIDELINES

In any circuit where accuracy is important, careful consideration

of the power supply and ground return layout helps to ensure

the rated performance. The printed circuit board on which the

AD5751 is mounted should be designed so that the AD5751 lies

on the analog plane.

The AD5751 should have ample supply bypassing of 10 μF in

parallel with 0.1 μF on each supply located as close to the

package as possible, ideally right up against the device. The 10

μF capacitors are the tantalum bead type. The 0.1 μF capacitor

should have low effective series resistance (ESR) and low

effective series inductance (ESI) such as the common ceramic

types, which provide a low impedance path to ground at high

frequencies to handle transient currents due to internal logic

switching.

The power supply lines of the AD5751 should use as large a

trace as possible to provide low impedance paths and reduce the

effects of glitches on the power supply line. Fast switching

signals such as clocks should be shielded with digital ground to

avoid radiating noise to other parts of the board and should

never be run near the reference inputs. A ground line routed

between the SDIN and SCLK lines helps reduce crosstalk

between them (not required on a multilayer board that has a

separate ground plane, but separating the lines helps). It is

essential to minimize noise on the REFIN line.

Avoid crossover of digital and analog signals. Traces on

opposite sides of the board should run at right angles to each

other. This reduces the effects of feed through the board. A

microstrip technique is by far the best, but not always possible

with a double-sided board. In this technique, the component

side of the board is dedicated to ground plane, while signal

traces are placed on the solder side.

GALVANICALLY ISOLATED INTERFACE

In many process control applications, it is necessary to provide

an isolation barrier between the controller and the unit being

controlled to protect and isolate the controlling circuitry from

any hazardous common-mode voltages that might occur. The

iCoupler® family of products from Analog Devices provides

voltage isolation in excess of 2.5 kV. The serial loading structure

of the AD5751 make it ideal for isolated interfaces because the

number of interface lines is kept to a minimum. Figure 16 shows

a 4-channel isolated interface to the AD5751 using an

ADuM1400. For further information, visit

http://www.analog.com/icouplers.

ENCODE

DECODE

ENCODE

DECODE

ENCODE

DECODE

VIA

VIB

VIC

VOA

VOB

VOC

ENCODE

DECODE

VID

VOD

ADuM1400 *

Controller

Serial Clock Out

Serial Data Out

SYNC Out

Control out

To SCLK

To SDIN

To SYNC

To CLEAR

*ADDITIONAL PINS OMITTED FOR CLARITY

Figure16. Isolated Interface

MICROPROCESSOR INTERFACING

Microprocessor interfacing to the AD5751 is via a serial bus that

uses protocol compatible with microcontrollers and DSP

processors. The communications channel is a 3-wire

(minimum) interface consisting of a clock signal, a data signal,

and a sync signal. The AD5751 require a 16-bit data-word with

data valid on the falling edge of SCLK.

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