AD9280-EB datasheet(24/25 Pages) AD

24 / 25 page

AD9280

–23–

GROUNDING AND LAYOUT RULES

As is the case for any high performance device, proper ground-

ing and layout techniques are essential in achieving optimal

performance. The analog and digital grounds on the AD9280

have been separated to optimize the management of return

currents in a system. Grounds should be connected near the

ADC. It is recommended that a printed circuit board (PCB) of

at least four layers, employing a ground plane and power planes,

be used with the AD9280. The use of ground and power planes

offers distinct advantages:

1. The minimization of the loop area encompassed by a signal

and its return path.

2. The minimization of the impedance associated with ground

and power paths.

3. The inherent distributed capacitor formed by the power plane,

PCB insulation and ground plane.

These characteristics result in both a reduction of electro-

magnetic interference (EMI) and an overall improvement in

performance.

It is important to design a layout that prevents noise from cou-

pling onto the input signal. Digital signals should not be run in

parallel with the input signal traces and should be routed away

from the input circuitry. Separate analog and digital grounds

should be joined together directly under the AD9280 in a solid

ground plane. The power and ground return currents must be

carefully managed. A general rule of thumb for mixed signal

layouts dictates that the return currents from digital circuitry

should not pass through critical analog circuitry.

DIGITAL OUTPUTS

Each of the on-chip buffers for the AD9280 output bits

(D0–D7) is powered from the DRVDD supply pins, separate

from AVDD. The output drivers are sized to handle a variety

of logic families while minimizing the amount of glitch energy

generated. In all cases, a fan-out of one is recommended to

keep the capacitive load on the output data bits below the speci-

fied 20 pF level.

For DRVDD = 5 V, the AD9280 output signal swing is com-

patible with both high speed CMOS and TTL logic families.

For TTL, the AD9280 on-chip, output drivers were designed to

support several of the high speed TTL families (F, AS, S). For

applications where the clock rate is below 32 MSPS, other TTL

families may be appropriate. For interfacing with lower voltage

CMOS logic, the AD9280 sustains 32 MSPS operation with

DRVDD = 3 V. In all cases, check your logic family data sheets

for compatibility with the AD9280 Digital Specification table.

THREE-STATE OUTPUTS

The digital outputs of the AD9280 can be placed in a high

impedance state by setting the THREE-STATE pin to HIGH.

This feature is provided to facilitate in-circuit testing or evaluation.

REV. E

AD9280-EB датащи(PDF) 24 Page - Analog Devices

AD9280-EB датащи(HTML) 24 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

датащи скачать

ссылки URL