LMH6580 Datasheet, PDF (16/24 Pages) National Semiconductor (TI) – 8x4 500MHz Analog Crosspoint Switch, Gain of 1, Gain of 2

Application Information
INTRODUCTION
The LMH6580/LMH6581 are high speed, fully buffered, non-
blocking, analog crosspoint switches. Having fully buffered
inputs allows the LMH6580/LMH6581 to accept signals from
low or high impedance sources without the worry of loading
the signal source. The fully buffered outputs will drive 75Ω or
50Ω back terminated transmission lines with no external com-
ponents other than the termination resistor. When disabled,
the outputs are in a high impedance state. The LMH6580/
LMH6581 can have any input connected to any (or all) output
(s). Conversely, a given output can have only one associated
input.
INPUT AND OUTPUT EXPANSION
The LMH6580/LMH6581 have high impedance inactive
states for both inputs and outputs allowing maximum flexibility
for crosspoint expansion. In addition the LMH6580/LMH6581
employ diagonal symmetry in pin assignments. The diagonal
symmetry makes it easy to use direct pin to pin vias when the
parts are mounted on opposite sides of a board. As an ex-
ample two LMH6580/LMH6581 chips can be combined on
one board to form either an 8 x 8 crosspoint or a 16 x 4 cross-
point. To make an 8 x 8 crosspoint all 8 input pins would be
tied together (Input 0 on side 1 to input 7 on side 2 and so on)
while the 4 output pins on each chip would be left separate.
To make the 16 x 4 crosspoint, the 4 outputs would be tied
together while all 16 inputs would remain independent. In the
16 x 4 configuration it is important not to have 2 connected
outputs active at the same time. With the 8 x 8 configuration,
on the other hand, having two connected inputs active is a
valid state. Crosspoint expansion as detailed above has the
advantage that the signal will go through only one crosspoint.
Expansion methods that have cascaded stages will suffer
bandwidth loss far greater than the small loading effect of
parallel expansion.
Output expansion as shown in Figure 1 is very straight for-
ward. Connecting the inputs of two crosspoint switches has a
very minor impact on performance. Input expansion requires
more planning. Input expansion, as show in Figure 2 and
Figure 3 gives the option of two ways to connect the outputs
of the crosspoint switches. In Figure 2 the crosspoint switch
outputs are connected directly together and share one termi-
nation resistor. This is the easiest configurarion to implement
and has only one drawback. Because the disabled output of
the unused crosspoint (only one output can be active at a
time) has a small amount of capacitance, the frequency re-
sponse of the active crosspoint will show peaking. This is
illustrated in Figure 4 and Figure 5. In most cases this small
amount of peaking is not a problem.
As illustrated in Figure 3 each crosspoint output can be given
its own termination resistor. This results in a frequency re-
sponse nearly identical to the non expansion case. There is
one drawback for the gain of 2 crosspoint, and that is gain
error. With a 75Ω termination resistor the 1250Ω resistance
of the disabled crosspoint output will cause a gain error. In
order to counter act this the termination resistors of both
crosspoints should be adjusted to approximately 80Ω. This
will provide very good matching, but the gain accuracy of the
system will now be dependent on the process variations of
the crosspoint resistors which have a variability of approxi-
mately ±20%.
FIGURE 1. Output Expansion
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FIGURE 2. Input Expansion with Shared Termination
Resistors
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