ADV7120

REV. B

–11–

Power Planes

The PC board layout should have two distinct power planes,

one for analog circuitry and one for digital circuitry. The analog

sociated analog circuitry. This power plane should be connected

a ferrite bead, as illustrated in Figure 8. This bead should be lo-

cated within three inches of the ADV7120.

The PCB power plane should provide power to all digital logic

on the PC board, and the analog power plane should provide

power to all ADV7120 power pins, voltage reference circuitry

and any output amplifiers.

The PCB power and ground planes should not overlay portions

of the analog power plane. Keeping the PCB power and ground

planes from overlaying the analog power plane will contribute to

a reduction in plane-to-plane noise coupling.

Supply Decoupling

Noise on the analog power plane can be further reduced by the

use of multiple decoupling capacitors. (See Figure 8.)

Optimum performance is achieved by the use of 0.1

µF ceramic

decoupled to ground. This should be done by placing the ca-

pacitors as close as possible to the device with the capacitor

leads as short as possible, thus minimizing lead inductance.

It is important to note that while the ADV7120 contains cir-

cuitry to reject power supply noise, this rejection decreases with

frequency. If a high frequency switching power supply is used,

the designer should pay close attention to reducing power sup-

ply noise. A dc power supply filter (Murata BNX002) will pro-

vide EMI suppression between the switching power supply and

the main PCB. Alternatively, consideration could be given to

using a three terminal voltage regulator.

Digital Signal Interconnect

The digital signal lines to the ADV7120 should be isolated as

much as possible from the analog outputs and other analog cir-

cuitry. Digital signal lines should not overlay the analog power

plane.

Due to the high clock rates used, long clock lines to the

ADV7120 should be avoided so as to minimize noise pickup.

Any active pull-up termination resistors for the digital inputs

not the analog power plane.

Analog Signal Interconnect

The ADV7120 should be located as close as possible to the out-

put connectors thus minimizing noise pickup and reflections

due to impedance mismatch.

The video output signals should overlay the ground plane, and

not the analog power plane, thereby maximizing the high fre-

quency power supply rejection.

For optimum performance, the analog outputs should each have

a source termination resistance to ground of 75

Ω (doubly ter-

minated 75

Ω configuration). This termination resistance should

be as close as possible to the ADV7120 so as to minimize

reflections.

Additional information on PCB design is available in an applica-

tion note entitled “Design and Layout of a Video Graphics Sys-

tem for Reduced EMI.” This application note is available from

Analog Devices, publication number E1309-15-10/89.