Preliminary Data Sheet

AD45335

APPLICATIONS INFORMATION

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Rev. PrA | 16 of 17

POWER SUPPLY DECOUPLING

RECOMMENDATIONS

On the AD45335, it is recommended to tie all grounds together as

close to the device as possible. If the number of supplies must be

reduced, bring all supplies back separately and make a provision

on the board via a link option to drive the AVCC and V+ pins from

the same supply. Decouple all power supplies adequately with 10

µF tantalum capacitors and 0.1 µF ceramic capacitors.

GUIDELINES FOR PCB LAYOUT

Design printed circuit boards such that the analog and digital

sections are separated and confined to the designated analog and

digital sections of the board. This facilitates the use of ground

planes that can be separated easily. A minimum etch technique is

generally the best for ground planes because it optimizes shielding

of sensitive signal lines. Join digital and analog ground planes in

one place only, at the AGND and DGND pins of the high resolution

converter. To isolate the high frequency bus of the processor from

the bus of the high resolution converters, buffer or latch data

and address buses on the board. These act as a Faraday shield

and increase the signal-to-noise performance of the converters

by reducing the amount of high frequency digital coupling. Avoid

running digital lines under the device because they couple noise

onto the die. Allow the ground plane to run under the IC to avoid

noise coupling.

Use as large a trace as possible for the supply lines of the device

to provide low impedance paths and reduce the effects of glitches

on the power supply line. Shield components, such as clocks with

fast-switching signals, with digital ground to avoid radiating noise to

other sections of the board. Never run clock signals near the analog

inputs of the device. Avoid crossovers of digital and analog signals.

Keep traces for analog inputs as wide and short as possible and

shield with analog ground if possible. Run traces on opposite sides

of the 2-layer PCB at right angles to each other to reduce the

effects of feedthrough through the board.

A microstrip technique is by far the best, but it is not always

possible to use with a double-sided board. In this technique,

the component side of the board is dedicated to ground planes,

and signals are placed on the solder side. Multilayer printed circuit

boards with dedicated ground, power, and tracking layers offer the

optimum solution in terms of obtaining analog performance, but at

increased manufacturing costs.

Good decoupling is vitally important when using high resolution

converters. Decouple all analog supplies with 10 µF tantalum ca-

pacitors in parallel with 0.1 µF ceramic capacitors to analog ground.

To achieve the best results from the decoupling components, place

them as close to the device as possible, ideally right up against the

IC or the IC socket. The main aim of a bypassing element is to

maximize the charge stored in the bypass loop while simultaneous-

ly minimizing the inductance of this loop. Inductance in the loop

acts as an impedance to high frequency transients and results in

power supply spiking. By keeping the decoupling as close to the

device as possible, the loop area is kept as small as possible,

thereby reducing the possibility of power supply spikes. Decouple

digital supplies of high resolution converters with 10 µF tantalum

capacitors and 0.1 µF ceramic capacitors to the digital ground

plane. Decouple the V+ supply with a 10 µF tantalum capacitor and

a 0.1 µF ceramic capacitor to AGND.

Decouple all logic chips with 0.1 µF ceramic capacitors to digital

ground to decouple high frequency effects associated with digital

circuitry.