ADA4939-1/ADA4939-2

Rev. 0 | Page 22 of 24

LAYOUT, GROUNDING, AND BYPASSING

As a high speed device, the ADA4939 is sensitive to the

PCB environment in which it operates. Realizing its superior

performance requires attention to the details of high speed

PCB design. This section shows a detailed example of how the

ADA4939-1 was addressed.

The first requirement is a solid ground plane that covers as

much of the board area around the ADA4939-1 as possible.

cleared of all ground and power planes (see Figure 51). 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 four-layer

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

The power supply pins should be bypassed as close to the device

as possible and directly to a nearby ground plane. High frequency

ceramic chip capacitors should be used. It is recommended that

two parallel bypass capacitors (1000 pF and 0.1 μF) be used for

each supply. The 1000 pF capacitor should be placed closer to

the device. Further away, low frequency bypassing should be

provided, using 10 μF tantalum capacitors from each supply

to ground.

Signal routing should be short and direct to avoid parasitic

effects. Wherever complementary signals exist, a symmetrical

layout should be provided to maximize balanced performance.

When routing differential signals over a long distance, PCB

traces should be close together, and any differential wiring

should be twisted 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 52. Recommended PCB Thermal Attach Pad Dimensions (Millimeters)

0.30

PLATED

VIA HOLE

1.30

GROUND PLANE

POWER PLANE

BOTTOM METAL

TOP METAL

Figure 53. Cross-Section of Four-Layer PCB Showing Thermal Via Connection to Buried Ground Plane (Dimensions in Millimeters)