AD8138ARZ-R7 datasheet(19/24 Pages) AD

19 / 24 page

Data Sheet

AD8138

Rev. G | Page 19 of 24

LAYOUT, GROUNDING, AND BYPASSING

As a high speed device, the AD8138 is sensitive to the PCB

environment in which it has to operate. Realizing its superior

specifications requires attention to various details of good high

speed PCB design.

The first requirement is for a good solid ground plane that

covers as much of the board area around the AD8138 as

possible. The only exception to this is that the two input pins

(Pin 1 and Pin 8) should be kept a few millimeters from the

ground plane, and ground should be removed from inner layers

and the opposite side of the board under the input pins. This

minimizes the stray capacitance on these nodes and helps

preserve the gain flatness vs. frequency.

The power supply pins should be bypassed as close as possible

to the device to the nearby ground plane. Good high frequency

ceramic chip capacitors should be used. This bypassing should

be done with a capacitance value of 0.01 µF to 0.1 µF for each

supply. Further away, low frequency bypassing should be provided

with 10 µF tantalum capacitors from each supply to ground.

The signal routing should be short and direct to avoid parasitic

effects. Wherever there are complementary signals, a symmetrical

layout should be provided to the extent possible to maximize

the balance performance. When running differential signals

over a long distance, the traces on the PCB should be close

together or any differential wiring should be twisted together to

minimize the area of the loop that is formed. This reduces the

radiated energy and makes the circuit less susceptible to

interference.

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