ADA4817-2ACPZ-R7 datasheet(23/29 Pages) AD

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Data Sheet

ADA4817-1/ADA4817-2

LAYOUT, GROUNDING, AND BYPASSING CONSIDERATIONS

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Rev. H | 23 of 29

create significant leakage. 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 it must

completely surround the input leads on all sides (above and below)

when 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. In addition, low absorption materials, such

as Teflon® or ceramic, can 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 picofarads of capacitance reduces the input impedance

at high frequencies, in turn increasing the gain of the amplifier,

causing peaking of the frequency response or even oscillations if

severe enough. It is recommended to place the external passive

components connected to the input pins 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.

INPUT-TO-INPUT/OUTPUT COUPLING

To minimize capacitive coupling between the inputs and outputs,

ensure that the output signal traces are not parallel with the inputs.

In addition, ensure that the input traces are not close to each other.

A minimum of 7 mils between the two inputs is recommended.

ADA4817-2ACPZ-R7 数据表(PDF) 23 Page - Analog Devices