Data Sheet

ADA4940-1/ADA4940-2

Rev. B | Page 27 of 32

LAYOUT, GROUNDING, AND BYPASSING

As a high speed device, the ADA4940-1/ADA4940-2 are

sensitive to the PCB environment in which they operate.

Realizing their superior performance requires attention to

the details of high speed PCB design.

ADA4940-1 LFCSP EXAMPLE

The first requirement is a solid ground plane that covers as

much of the board area around the ADA4940-1 as possible.

of all ground and power planes (see Figure 74). Clearing the

ground and power planes minimizes any stray capacitance at

these nodes and prevents peaking of the response of the

amplifier at high frequencies.

the exposed pad, soldered to a high thermal conductivity 4-layer

circuit board, as described in EIA/JESD 51-7.

Bypass the power supply pins as close to the device as possible

and directly to a nearby ground plane. Use high frequency ceramic

chip capacitors. Use two parallel bypass capacitors (1000 pF and

0.1 µF) for each supply. Place the 1000 pF capacitor closer to the

device. Further away, provide low frequency bypassing using

10 µF tantalum capacitors from each supply to ground.

Ensure that signal routing is short and direct to avoid parasitic

effects. Wherever complementary signals exist, provide a

symmetrical layout to maximize balanced performance. When

routing differential signals over a long distance, ensure that

PCB traces are close together, and twist any differential wiring

such that loop area is minimized. Doing this reduces radiated

energy and makes the circuit less susceptible to interference.

1.30

0.80

0.80

1.30

Figure 75. Recommended PCB Thermal Attach Pad Dimensions (mm)

0.30

PLATED

VIA HOLE

1.30

GROUND PLANE

POWER PLANE

BOTTOM METAL

TOP METAL

Figure 76. Cross-Section of 4-Layer PCB Showing Thermal Via Connection to Buried Ground Plane (Dimensions in mm)