LMH6560 datasheet(21/23 Pages) NSC

21 / 23 page

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 - 50

Ω for

most applications and 75

Ω 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 50

Ω 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 100

Ω, while measuring the

crosstalk output signal at buffer 3, which input was termi-

nated with a resistor of 50

Ω.

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

Evaluation board

Part Number

LMH6560MA

SOIC-14

CLC730145

LMH6560MT

TSSOP-14

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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