ADA4817-2ACPZ-R2 datasheet(17/24 Pages) AD

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ADA4817-1/ADA4817-2

Rev. 0 | Page 17 of 24

LEAKAGE CURRENTS

Poor PCB layout, contaminants, and the board insulator

material can create leakage currents that are much larger than

the input bias current of the ADA4817-1/ADA4817-2. Any

voltage differential between the inputs and nearby runs sets

up leakage currents through the PCB insulator, for example, 1 V/

100 GΩ = 10 pA. Similarly, any contaminants, such as skin oils

on the board, can 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 should completely

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

while 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 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.

INPUT-TO-INPUT/OUTPUT COUPLING

To minimize capacitive coupling between the inputs and

outputs, the output signal traces should not be parallel with

the inputs. In addition, the input traces should not be very

close to each other. A minimum of 7 mils between the two

inputs is recommended.

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