LMH6582_0708 Datasheet, PDF(10/16 Page) National Semiconductor (TI) – 16x8 550MHz Analog Crosspoint Switch, Gain of 1

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LMH6582_0708 Datasheet, PDF (10/16 Pages) National Semiconductor (TI) – 16x8 550MHz Analog Crosspoint Switch, Gain of 1

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FIGURE 3. Input Expansion with Separate Termination

Resistors

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FIGURE 4. Input Expansion Frequency Response with

Direct Connection and Isolation Resistors

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FIGURE 5. Input Expansion Frequency Response

DRIVING CAPACITIVE LOADS

Capacitive output loading applications will benefit from the

use of a series output resistor ROUT. Capacitive loads of 5 pF

to 120 pF are the most critical, causing ringing, frequency re-

sponse peaking and possible oscillation. The chart âSuggest-

ed ROUT vs. Cap Loadâ gives a recommended value for

selecting a series output resistor for mitigating capacitive

loads. The values suggested in the charts are selected for 0.5

dB or less of peaking in the frequency response. This gives a

good compromise between settling time and bandwidth. For

applications where maximum frequency response is needed

and some peaking is tolerable, the value of ROUT can be re-

duced slightly from the recommended values. When driving

transmission lines the 50â¦ or 75â¦ matching resistor makes

the series output resistor unnecessary.

USING OUTPUT BUFFERING TO ENHANCE BANDWIDTH

AND INCREASE RELIABILITY

The LMH6582 crosspoint switch can offer enhanced band-

width and reliability with the use of external buffers on the

outputs. The bandwidth is increased by unloading the outputs

and driving the high impedance of an external buffer. See the

Frequency Response 1 kâ¦ Load curve in the Typical Perfor-

mance section for an example of bandwidth achieved with

less loading on the outputs. For this technique to provide

maximum benefit a very high speed amplifier such as the

LMH6703 should be used. As shown in Figure 6 there is an

optional resistor ROUT between the LMH6582 and the buffer

input. This resistor will isolate the amplifier input capacitance

and board capacitance from the crosspoint switch output. Any

traces longer than 1 cm will most likely require some termi-

nation resistance as shown.

Besides offering enhanced bandwidth performance, using an

external buffer provides for greater system reliability. The first

advantage is to reduce thermal loading on the crosspoint

switch. This reduced die temperature which increases the life

of the crosspoint. The second advantage is enhanced ESD

reliability. It is impossible to build high speed devices that can

withstand all possible ESD events. With external buffers the

crosspoint switch is isolated from ESD events on the external

system connectors.

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