AD8028WARMZ-R7 datasheet(23/27 Pages) AD

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Data Sheet

AD8027/AD8028

WIDEBAND OPERATION

analog.com

Rev. E | 23 of 27

PCB LAYOUT

As with all high speed op amps, achieving optimum performance

from the AD8027/AD8028 requires careful attention to PCB layout.

Particular care must be exercised to minimize lead lengths of

the bypass capacitors. Excess lead inductance can influence the

frequency response and even cause high frequency oscillations.

The use of a multilayer board with an internal ground plane can

reduce ground noise and enable a tighter layout.

To achieve the shortest possible lead length at the inverting input,

position the feedback resistor, RF, beneath the board so that it

spans the distance from the output, to the inverting input. Situate

the return node of the resistor, RG, as closely as possible to the

return node of the negative supply bypass capacitor.

On multilayer boards, clear all layers underneath the op amp

of metal to avoid creating parasitic capacitive elements. This is

especially true at the summing junction (the negative input). Extra

capacitance at the summing junction can cause increased peaking

in the frequency response and lower phase margin.

GROUNDING

To minimize parasitic inductances and ground loops in high speed,

densely populated boards, a ground plane layer is critical. Under-

standing where the current flows in a circuit is critical in the

implementation of high speed circuit design. The length of the

current path is directly proportional to the magnitude of the parasitic

inductances and, therefore, the high frequency impedance of the

path. Fast current changes in an inductive ground return can create

unwanted noise and ringing.

The length of the high frequency bypass capacitor pads and traces

is critical. A parasitic inductance in the bypass grounding works

against the low impedance created by the bypass capacitor. Be-

cause load currents flow from supplies as well as ground, place the

load at the same physical location as the bypass capacitor ground.

For large values of capacitors, which are intended to be effective at

lower frequencies, the current return path length is less critical.

POWER SUPPLY BYPASSING

Power supply pins are actually inputs, and care must be taken to

provide a clean, low noise, dc voltage source to these inputs. The

bypass capacitors have two functions.

►

Provide a low impedance path for unwanted frequencies from the

supply inputs to ground, thereby reducing the effect of noise on

the supply lines.

►

Provide sufficient localized charge storage, for fast switching

conditions and minimizing the voltage drop at the supply pins

and the output of the amplifier. This is usually accomplished with

larger electrolytic capacitors.

Decoupling methods are designed to minimize the bypassing impe-

dance at all frequencies. This can be accomplished with a combina-

tion of capacitors in parallel to ground.

Use high quality ceramic chip capacitors and always keep them as

close as possible to the amplifier package. A parallel combination of

a 0.01 µF ceramic and a 10 µF electrolytic covers a wide range of

rejection for unwanted noise. The 10 µF capacitor is less critical for

high frequency bypassing, and, in most cases, one per supply line

is sufficient.

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