ADA4817-2ACPZ-R2 datasheet(18/25 Pages) AD

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ADA4817-1/ADA4817-2

Data Sheet

Rev. C | Page 18 of 25

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 it must 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 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.

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