LMH6559_06 Datasheet, PDF (20/22 Pages) National Semiconductor (TI) – High-Speed, Closed-Loop Buffer

Application Notes (Continued)
design have at least one ground plane on a pcb which
provides a low impedance path for all decoupling capacitors
and other ground connections. Care should be taken espe-
cially that on- board transmission lines have the same im-
pedance as the cables to which they are connected - 50Ω for
most applications and 75Ω in case of video and cable TV
applications. Such transmission lines usually require much
wider traces on a standard double sided PCB board than
needed for a ’normal’ trace. Another important issue is that
inputs and outputs must not ’see’ each other. This occurs if
inputs and outputs are routed together over the pcb with only
a small amount of physical separation, particularly when
there is a high differential in signal level between them.
Furthermore components should be placed as flat and low
as possible on the surface of the PCB. For higher frequen-
cies a long lead can act as a coil, a capacitor or an antenna.
A pair of leads can even form a transformer. Careful design
of the pcb avoids oscillations or other unwanted behaviors.
For ultra high frequency designs only surface mount compo-
nents will give acceptable results. (for more information see
OA-15).
NSC suggests the following evaluation boards as a guide for
high frequency layout and as an aid in device testing and
characterization.
Device
Package
LMH6559MA
LMH6559MAX
LMH6559MF
LMH6559MFX
SOIC-8
SOIC-8
SOT23-5
SOT23-5
Evaluation Board
Part Number
CLC730245
CLC730245
CLC730136
CLC730136
These free evaluation boards are shipped when a device
sample request is placed with National Semiconductor.
POWER SEQUENCING OF THE LMH6559
Caution should be exercised in applying power to the
LMH6559. When the negative power supply pin is left float-
ing it is recommended that other pins, such as positive
supply and signal input should also be left unconnected. If
the ground is floating while other pins are connected the
input circuitry is effectively biased to ground, with a mostly
low ohmic resistor, while the positive power supply is ca-
pable of delivering significant current through the circuit. This
causes a high input bias current to flow which degrades the
input junction. The result is an input bias current which is out
of specification. When using inductive relays in an applica-
tion care should be taken to connect first both power con-
nections before connecting the bias resistor to the input.
www.national.com
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