AD5764CSUZ datasheet(27/28 Pages) AD

27 / 28 page

Data Sheet

AD5764

Rev. F | Page 27 of 28

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 PCB on which the AD5764 is mounted

must be designed so that the analog and digital sections are sepa-

rated and confined to certain areas of the board. If the AD5764 is

in a system where multiple devices require an AGND-to-DGND

connection, the connection is to be made at one point only. The

star ground point is established as close as possible to the device.

The AD5764 must 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 must 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 AD5764 must be 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, must be shielded with digital ground to avoid radiating

noise to other parts of the board, and must 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, which has a separate ground plane; however,

it is helpful to separate the lines). It is essential to minimize noise

on the reference inputs because it couples through to the DAC

output. Avoid crossover of digital and analog signals. Traces on

opposite sides of the board must run at right angles to each other.

This reduces the effects of feedthrough on the board. A microstrip

technique is recommended, but not always possible with a double-

sided board. In this technique, the component side of the board

is dedicated to the ground plane, and 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 may occur. Isocoup-

lers provide voltage isolation in excess of 2.5 kV. The serial

loading structure of the AD5764 makes it ideal for isolated

interfaces because the number of interface lines is kept to a

minimum. Figure 37 shows a 4-channel isolated interface to the

AD5764 using an ADuM1400. For more information, go to

www.analog.com.

VIA

SERIAL CLOCK

OUT

TO SCLK

VOA

ENCODE

DECODE

VIB

SERIAL DATA

OUT

TO SDIN

VOB

ENCODE

DECODE

VIC

SYNC OUT

VOC

ENCODE

DECODE

VID

CONTROL OUT

VOD

ENCODE

DECODE

MICROCONTROLLER

ADuM1400*

*ADDITIONAL PINS OMITTED FOR CLARITY.

TO SYNC

TO LDAC

Figure 37. Isolated Interface

MICROPROCESSOR INTERFACING

Microprocessor interfacing to the AD5764 is via a serial bus

that uses a standard protocol that is compatible with micro-

controllers and DSP processors. The communications channel

is a 3-wire (minimum) interface consisting of a clock signal, a

data signal, and a synchronization signal. The AD5764 requires

a 24-bit data-word with data valid on the falling edge of SCLK.

For all the interfaces, the DAC output update can be performed

automatically when all the data is clocked in, or it can be done

under the control of LDAC. The contents of the data register

can be read using the readback function.

EVALUATION BOARD

The AD5764 comes with a full evaluation board to aid designers

in evaluating the high performance of the part with minimum

effort. All that is required with the evaluation board is a power

supply and a PC. The AD5764 evaluation kit includes a populated,

tested AD5764 PCB. The evaluation board interfaces to the USB

port of the PC. Software is available with the evaluation board,

which allows the user to easily program the AD5764. The software

runs on any PC that has Microsoft® Windows® 2000/NT/XP

installed.

The EVAL-AD5764EB data sheet is available, which gives full

details on the operation of the evaluation board.

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