LMH6401_16 Datasheet, PDF (37/48 Pages) Texas Instruments – LMH6401 DC to 4.5 GHz, Fully-Differential, Digital Variable-Gain Amplifier

www.ti.com
12 Layout
LMH6401
SBOS730A – APRIL 2015 – REVISED MAY 2015
12.1 Layout Guidelines
When dealing with a device with relatively high gain and bandwidth in excess of 1 GHz, certain board layout
precautions must be taken to ensure stability and optimum performance. TI recommends that the LMH6401
board be multi-layered to improve thermal performance, grounding, and power-supply decoupling. The differential
input and output traces must be symmetrical in order to achieve the best linearity performance.
By sandwiching the power-supply layer between ground layers on either side (with thin dielectric thicknesses),
parasitic capacitance between power and ground functions as a distributed, high-resonance frequency capacitor
to help with power-supply decoupling. The LMH6401 evaluation board includes a total of six layers and the
positive (VS+) and negative (VS–) power planes are sandwiched in the middle with a board stack-up (dielectric
thickness), as shown in Figure 71, to help with supply decoupling. Both VS+ and VS– must be connected to the
internal power planes through multiple vias in the immediate vicinity of the supply pins. In addition, low ESL,
ceramic, 0.01-μF decoupling capacitors to the supplies are placed on the same layer as the device to provide
supply decoupling.
Routing high-frequency signal traces on a PCB requires careful attention to maintain signal integrity. A board
layout software package can simplify the trace thickness design to maintain impedances for controlled
impedance signals. In order to isolate the affect of board parasitic on frequency response, TI recommends
placing the external output matching resistors close to the amplifier output pins. A 0.01-µF bypass capacitor is
also recommended close to the VOCM pins to suppress high-frequency common-mode noise. Refer to the user
guide LMH6401EVM Evaluation Module (SLOU406) for more details on board layout and design.
In order to improve board mechanical reliability, the LMH6401 has square anchor pins on four corners of the
package that must be soldered to the board for mechanical strength.
L1 - Top
L2 - GND
L3 ± VS+
L4 ± VS-
L5 ± GND
L6 - Bottom
0.0166"
0.005"
0.0482"
0.005"
0.0166"
1-oz. copper on all layers,
100-ŸGLIIHUHQWLDOWUDFHLPSHGDQFHRQWKHWRSOD\HU,
and Rogers 4350 dielectric on the top layer.
Figure 71. Recommended PCB Layer Stack-Up for a Six-Layer Board
Copyright © 2015, Texas Instruments Incorporated
Product Folder Links: LMH6401
Submit Documentation Feedback
37