ADA4939-1/ADA4939-2

Data Sheet

Rev. A | Page 22 of 24

LAYOUT, GROUNDING, AND BYPASSING

As a high speed device, the ADA4939-1 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.

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

(RG), and the input summing nodes (Pin 2 and Pin 3) must be

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 thermal resistance, θJA, is specified for the device, including

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

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

Figure 51. 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 to 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 the

PCB traces are close together, and twist any differential wiring

such that the loop area is minimized which 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 4-Layer PCB Showing Thermal Via Connection to Buried Ground Plane (Dimensions in Millimeters)