LMH6584 Datasheet, PDF (12/20 Pages) National Semiconductor (TI) – 32x16 400 MHz Analog Crosspoint Switches, Gain of 1, Gain of 2

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FIGURE 3. Input Expansion with Separate Termination
Resistors
DRIVING CAPACITIVE LOADS
Capacitive output loading applications will benefit from the
use of a series output resistor R_OUT. Capacitive loads of
5 pF to 120 pF are the most critical, causing ringing, frequency
response peaking and possible oscillation. As starting values,
a capacitive load of 5 pF should have around 75 Ω of isolation
resistance. A value of 120 pF would require around 12Ω.
When driving transmission lines the 50Ω or 75Ω matching re-
sistor normally provides enough isolation.
USING OUTPUT BUFFERING TO ENHANCE RELIABILITY
The LMH6584/LMH6585 crosspoint switch can offer en-
hanced reliability with the use of external buffers 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 4 .
The advantage offered by using external buffers is to reduce
thermal loading on the crosspoint switch. This reduced die
temperature will increase the life of the crosspoint. Another
advantage is enhanced ESD reliability. It is very difficult 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.
In the example in Figure 4 the resistor R_L is required to provide
a load for the crosspoint output buffer. Without R_Lexcessive
frequency response peaking is likely and settling times of
transient signals will be poor. As the value of R_L is reduced
the bandwidth will also go down. The amplifier shown in the
example is an LMH6703 this amplifier offers high speed and
flat bandwidth. Another suitable amplifier is the LMH6702.
The LMH6702 is a faster amplifier that can be used to gen-
erate high frequency peaking in order to equalize longer cable
lengths. If board space is at a premium the LMH6739 or the
LMH6734 are triple selectable gain buffers which require no
external resistors.
CROSSTALK
When designing a large system such as a video router,
crosstalk can be a very serious problem. Extensive testing in
our lab has shown that most crosstalk is related to board lay-
out rather than the crosspoint switch. There are many ways
to reduce board related crosstalk. Using controlled
impedance lines is an important step. Using well decoupled
power and ground planes will help as well. When crosstalk
does occur within the crosspoint switch itself it is often due to
signals coupling into the power supply pins. Using appropriate
supply bypassing will help to reduce this mode of coupling.
Another suggestion is to place as much grounded copper as
possible between input and output signal traces. Care must
be taken, though, not to influence the signal trace impedances
by placing shielding copper too closely. One other caveat to
consider is that as shielding materials come closer to the sig-
nal trace the trace needs to be smaller to keep the impedance
from falling too low. Using thin signal traces will result in un-
acceptable losses due to trace resistance. This effect be-
comes even more pronounced at higher frequencies due to
the skin effect. The skin effect reduces the effective thickness
of the trace as frequency increases. Resistive losses make
crosstalk worse because as the desired signal is attenuated
with higher frequencies crosstalk increases at higher frequen-
cies.
DIGITAL CONTROL
Block Diagram
FIGURE 4. Buffered Output
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FIGURE 5. Block Diagram
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The LMH6584/LMH6585 has internal control registers that
store the programming states of the crosspoint switch. The
logic is two staged to allow for maximum programming flexi-
bility. The first stage of the control logic is tied directly to the
crosspoint switching matrix. This logic consists of one register
for each output that stores the on/off state and the address of
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