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AD9834

Data Sheet

GROUNDING AND LAYOUT

The printed circuit board (PCB) that houses the AD9834 should

be designed so that the analog and digital sections are separated

and confined to certain areas of the board. This facilitates the

use of ground planes that can easily be separated. A minimum

etch technique is generally best for ground planes because it

gives the best shielding. Digital and analog ground planes

should only be joined in one place. If the AD9834 is the only

device requiring an AGND-to-DGND connection, the ground

planes should be connected at the AGND and DGND pins of

the AD9834. If the AD9834 is in a system where multiple devices

require AGND-to-DGND connections, the connection should

be made at one point only, establishing a star ground point as

close as possible to the AD9834.

Avoid running digital lines under the device because these

couple noise onto the die. The analog ground plane should be

allowed to run under the AD9834 to avoid noise coupling. The

power supply lines to the AD9834 should use as large a track 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 sections of the board. Avoid crossover of digital

and analog signals. Traces on opposite sides of the board should

run at right angles to each other to reduce the effects of feed-

through through the board. A microstrip technique is by far the

best, but it is not always possible 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 other side.

Good decoupling is important. The analog and digital supplies

to the AD9834 are independent and separately pinned out to

minimize coupling between analog and digital sections of the

device. All analog and digital supplies should be decoupled to

AGND and DGND, respectively, with 0.1 μF ceramic capacitors

in parallel with 10 μF tantalum capacitors. To achieve the best

performance from the decoupling capacitors, they should be

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

the device. In systems where a common supply is used to drive

both the AVDD and DVDD of the AD9834, it is recommended

that the system’s AVDD supply be used. This supply should have

the recommended analog supply decoupling between the AVDD

pins of the AD9834 and AGND, and the recommended digital

supply decoupling capacitors between the DVDD pins and DGND.

Proper operation of the comparator requires good layout strategy.

The strategy must minimize the parasitic capacitance between

VIN and the SIGN BIT OUT pin by adding isolation using a

ground plane. For example, in a multilayered board, the VIN

signal could be connected to the top layer, and the SIGN BIT OUT

could be connected to the bottom layer so that isolation is

provided by the power and ground planes between them.

Rev. D | Page 26 of 32

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