LMH6881 Datasheet, PDF (27/35 Pages) Texas Instruments – Small Signal Bandwidth: 2400 MHz

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LMH6881
SNOSC72F – JUNE 2012 – REVISED FEBRUARY 2015
9 Power Supply Recommendations
The LMH6881 was designed to be operated on 5-V power supplies. The voltage range for VCC is from 4.75 V to
5.25 V. Power-supply accuracy of 5% or better is advised. When operated on a board with high-speed digital
signals it is important to provide isolation between digital signal noise and the analog input pins. The
SP16160CH1RB reference board provides an example of good board layout.
The power supply pins are 19, 20, 23 and 24. Each supply pin should be decoupled with a low-inductance,
surface-mount ceramic capacitor of approximately 10 nF as close to the device as possible. When vias are used
to connect the bypass capacitors to a ground plane the vias should be configured for minimal parasitic
inductance. One method of reducing via inductance is to use multiple vias. For broadband systems two
capacitors per supply pin are advised.
To avoid undesirable signal transients the LMH6881 should not be powered on with large inputs signals present.
Careful planning of system power on sequencing is especially important to avoid damage to ADC inputs when an
ADC is used in the application.
10 Layout
10.1 Layout Guidelines
It is very important to employ good high-speed layout techniques when dealing with devices having relatively
high gain bandwidth in excess of 1 GHz to ensure stability and optimum performance. The LMH6881 evaluation
board provides a good reference for suggested layout techniques. The LMH6881 evaluation board was designed
for both good signal integrity and thermal dissipation using higher performance (Rogers) dielectric on the top
layer. The high performance dielectric provides well matched impedance and low loss to frequencies beyond 1
GHz.
TI recommends that the LMH6881 board be multi-layered to improve thermal performance, grounding and
power-supply decoupling. The LMH6881 evaluation board is an 8-layered board with the supply sandwiched in-
between the GND layers for decoupling and having the stack up as Top layer - GND - GND - GND - Supply -
GND - GND - Bottom layer. All signal paths are routed on the top layer on the higher performance (Rogers)
dielectric, while the remainder signal layers are conventional FR4.
10.1.1 Uncontrolled Impedance Traces
It is important to pay careful attention while routing high-frequency signal traces on the PCB to maintain signal
integrity. A good board layout software package can simplify the trace thickness design to maintain controlled
characteristic impedances for high-frequency signals. Eliminating copper (the ground and power plane) from
underneath the input and output pins of the device also helps in minimizing parasitic capacitance affecting the
high-frequency signals near the PCB and package junctions. The LMH6881 evaluation board has copper keep-
out areas under both the input and the output traces for this purpose. It is recommended that the application
board also follow these keep-out areas to avoid any performance degradation.
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