AD8065WARTZ-R7 datasheet(23/28 Pages) AD

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Data Sheet

AD8065/AD8066

Rev. L | Page 23 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.

LEAKAGE CURRENTS

Poor PC board layout, contaminants, and the board insulator

material can create leakage currents that are much larger than

the input bias current of the AD8065/AD8066. Any voltage

differential between the inputs and nearby runs sets up leakage

currents through the PC board insulator, for example, 1 V/100 GΩ

= 10 pA. Similarly, any contaminants on the board can create

significant leakage (skin oils are a common problem). To reduce

leakage significantly, put a guard ring (shield) around the inputs

and input leads that are driven to the same voltage potential as

the inputs. This way there is no voltage potential between the

inputs and surrounding area to set up any leakage currents.

For the guard ring to be completely effective, it must be driven

by a relatively low impedance source and should completely

surround the input leads on all sides, above and below, using

a multilayer board.

Another effect that can cause leakage currents is the charge

absorption of the insulator material itself. Minimizing the

amount of material between the input leads and the guard ring

helps to reduce the absorption. Also, low absorption materials,

such as Teflon® or ceramic, could be necessary in some instances.

INPUT CAPACITANCE

Along with bypassing and ground, high speed amplifiers can be

sensitive to parasitic capacitance between the inputs and ground.

A few pF of capacitance reduces the input impedance at high

frequencies, in turn increasing the amplifier’s gain, causing peaking

of the frequency response or even oscillations, if severe enough.

It is recommended that the external passive components connected

to the input pins be placed as close as possible to the inputs to

avoid parasitic capacitance. The ground and power planes must

be kept at a small distance from the input pins on all layers of

the board.

OUTPUT CAPACITANCE

To a lesser extent, parasitic capacitances on the output can cause

peaking and ringing of the frequency response. There are two

methods to effectively minimize their effect:

•

As shown in Figure 57, put a small value resistor (RS) in

series with the output to isolate the load capacitor from the

amp’s output stage. A good value to choose is 20 Ω (see

Figure 10).

•

Increase the phase margin with higher noise gains or add

a pole with a parallel resistor and capacitor from −IN to

the output.

RS = 20Ω

AD8065

Figure 57. Output Isolation Resistor

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