LMH6560 Datasheet, PDF (21/23 Pages) National Semiconductor (TI) – Quad, 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 imped-
ance 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. If
routed close together crosstalk will occur and in that case a
small amount of the original signal will appear at the other
trace. The same effect will occur internally in the device. This
means that signal is jumping over from one buffer to the
other producing a part of the signal of buffer one in the other
buffers. To improve crosstalk performance it is recom-
mended to use a grounded guard-trace between signal lines
and to ground unused pins from the device package.
Crosstalk becomes more and more noticeable for the higher
frequencies. For frequencies below 1MHz crosstalk has a
signal level as low as −70dB below the incoming signal. For
higher frequencies crosstalk will degrade until about −35dB
at 100MHz. (see typical performance characteristics) The
best way to see this, is applying a pulse to one of the buffers
and looking at the output of one of the others. The flat portion
of such a pulse represents the lowest frequencies which are
highly suppressed and the edge of the incoming pulse rep-
resenting the highest frequencies will appear at the output.
For reducing the effect of crosstalk it is recommended to
terminate unused inputs and outputs with a low ohmic resis-
tor such as 50Ω for an input or 100Ω for an output to ground.
While measuring the crosstalk, signal was applied to buffer 2
which output was terminated with 100Ω, while measuring the
crosstalk output signal at buffer 3, which input was termi-
nated with a resistor of 50Ω.
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
LMH6560MA
LMH6560MT
Package
SOIC-14
TSSOP-14
Evaluation board
Part Number
CLC730145
CLC730132
These free evaluation boards are shipped when a device
sample request is placed with National Semiconductor.
POWER SEQUENCING OF THE LMH6560
Caution should be used in applying power to the LMH6560.
When the negative power supply pin is left floating 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 resis-
tor, while the positive power supply is capable 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 specifica-
tion. When using inductive relays in an application care
should be taken to connect first both power connections
before connecting the bias resistor to the input.
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