AD8065ARTZ-R2 datasheet(21/28 Pages) AD

21 / 28 page

AD8065/AD8066

Rev. G | Page 21 of 28

LAYOUT, GROUNDING, AND BYPASSING CONSIDERATIONS

POWER SUPPLY BYPASSING

Power supply pins are actually inputs and care must be taken so

that a noise-free stable dc voltage is applied. The purpose of

bypass capacitors is to create low impedances from the supply

to ground at all frequencies, thereby shunting or filtering most

of the noise.

Decoupling schemes are designed to minimize the bypassing

impedance at all frequencies with a parallel combination of

capacitors. 0.1 μF (X7R or NPO) chip capacitors are critical and

should be as close as possible to the amplifier package. The

4.7 μF tantalum capacitor is less critical for high frequency

bypassing, and, in most cases, only one is needed per board at

the supply inputs.

GROUNDING

A ground plane layer is important in densely packed PC boards

to spread the current minimizing parasitic inductances.

However, an understanding of where the current flows in a

circuit is critical to implementing effective high speed circuit

design. The length of the current path is directly proportional to

the magnitude of parasitic inductances and, therefore, the high

frequency impedance of the path. High speed currents in an

inductive ground return create unwanted voltage noise.

The length of the high frequency bypass capacitor leads is most

critical. A parasitic inductance in the bypass grounding works

against the low impedance created by the bypass capacitor.

Place the ground leads of the bypass capacitors at the same

physical location. Because load currents flow from the supplies

as well, the ground for the load impedance should be at the

same physical location as the bypass capacitor grounds. For the

larger value capacitors, which are effective at lower frequencies,

the current return path distance is less critical.

AD8065ARTZ-R2 数据表(PDF) 21 Page - Analog Devices