AD5749 datasheet(25/28 Pages) AD

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AD5749

Rev. 0 | Page 25 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 AD5749 is

mounted should be designed so that the AD5749 lies on the

analog plane.

The AD5749 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 capaci-

tor 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.

In systems where there are many devices on one board, it is often

useful to provide some heat sinking capability to allow the power

to dissipate easily.

AD5749

GND

PLANE

BOARD

Figure 36. Paddle Connection to Board

The AD5749 has an exposed paddle beneath the device.

Connect this paddle to the GND of the AD5749. For optimum

performance, special considerations should be used to design

the motherboard and to mount the package. For enhanced

thermal, electrical, and board level performance, the exposed

paddle on the bottom of the package should be soldered to the

corresponding thermal land paddle on the PCB (GND).

Thermal vias should be designed into the PCB land paddle area

to further improve heat dissipation.

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. The

iCoupler® family of products from Analog Devices, Inc., provides

voltage isolation in excess of 5.0 kV. The serial loading structure

of the AD5749 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 AD5749 using an ADuM1400.

For further information, visit http://www.analog.com/icouplers.

VIA

VOA

VIB

VOB

VIC

VOC

VID

VOD

ENCODE

DECODE

ENCODE

DECODE

ENCODE

DECODE

ENCODE

DECODE

SERIAL

CLOCK OUT

SERIAL

DATA OUT

SYNC OUT

CONTROL OUT

CONTROLLER

SCLK

SDIN

SYNC

CLEAR

ADuM14001

1ADDITIONAL PINS OMITTED FOR CLARITY.

Figure 37. Isolated Interface

MICROPROCESSOR INTERFACING

Microprocessor interfacing to the AD5749 is via a serial bus that

uses a protocol compatible with microcontrollers and DSP proces-

sors. The communication channel is a 3-wire (minimum)

interface consisting of a clock signal, a data signal, and a SYNC

signal. The AD5749 requires a 16-bit data-word with data valid

on the falling edge of SCLK.

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