Data Sheet

ADA4938-1/ADA4938-2

Rev. B | Page 23 of 26

LAYOUT, GROUNDING, AND BYPASSING

As high speed devices, the ADA4938-1/ADA4938-2 are

sensitive to the PCB environment in which it operates.

Realizing its superior performance requires attention to the

details of high speed PCB design.

The first requirement is a solid ground plane that covers as much of

the board area around the ADA4938-1/ADA4938-2 as possible.

However, the area near the feedback resistors (RF), input gain

resistors (RG), and the input summing nodes should be cleared

of all ground and power planes (see Figure 66). 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 thermal resistance, θJA, is specified for the device, including

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

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

is electrically isolated from the device; therefore, it can be con-

nected to a ground plane using vias. Examples of the thermal

attach pad and via structure for the ADA4938-1 are shown in

Figure 67 and Figure 68.

Figure 66. Ground and Power Plane Voiding in Vicinity of RF and RG

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. It is recommended that two parallel bypass capa-

citors (1000 pF and 0.1 μF) be used for each supply with the

1000 pF capacitor placed closer to the device; if further away,

provide low frequency bypassing 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, provide a

symmetrical layout to maximize balanced performance.

When routing differential signals over a long distance, keep

PCB traces close together and twist any differential wiring to

minimize loop area. Doing this reduces radiated energy and

makes the circuit less susceptible to interference.

1.30

0.80

0.80

1.30

Figure 67. Recommended PCB Thermal Attach Pad (ADA4938-1)

(Dimensions in mm)

0.30

PLATED

VIA HOLE

1.30

GROUND PLANE

POWER PLANE

BOTTOM METAL

TOP METAL

Figure 68. Cross-Section of a 4-Layer PCB (ADA4938-1) Showing a Thermal Via Connection to the Buried Ground Plane (Dimensions in mm)