LMH6584 Datasheet, PDF (11/20 Pages) National Semiconductor (TI) – 32x16 400 MHz Analog Crosspoint Switches, Gain of 1, Gain of 2

Application Information
INTRODUCTION
The LMH6584/LMH6585 are high speed, fully buffered, non
blocking, analog crosspoint switches. Having fully buffered
inputs allow the LMH6584/LMH6585 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 LMH6584/
LMH6585 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 LMH6584/LMH6585 have high impedance inactive
states for both inputs and outputs allowing maximum flexibility
for Crosspoint expansion. In addition the LMH6584/LMH6585
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 LMH6584/LMH6585 chips can be combined on
one board to form either an 32 x 32 crosspoint or a 64 x 16
crosspoint. To make a 32 x 32 cross-point all 32 input pins
would be tied together (Input 0 on side 1 to input 31 on side
2 and so on) while the 16 output pins on each chip would be
left separate. To make the 64 x 16 crosspoint, the 16 outputs
would be tied together while all 64 inputs would remain inde-
pendent. In the 64 x 16 configuration it is important not to have
two connected outputs active at the same time. With the 32 x
32 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 path has only one
crosspoint in it at a time. Expansion methods that have cas-
caded stages will suffer bandwidth loss far greater than the
small loading effect of parallel expansion.
Output expansion is very straight forward. Connecting the in-
puts of two crosspoint switches has a very minor impact on
performance. Input expansion requires more planning. As
show in Figure 1 and Figure 2 there are two ways to connect
the outputs of the crosspoint switches. In Figure 2 the cross-
point switch outputs are connected directly together and
share one termination resistor. This is the easiest configura-
tion 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 response of the active crosspoint will show peak-
ing.
As illustrated in Figure 1 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 counteract this the termination resistors of both cross-
points should be adjusted to approximately 71Ω. This will
provide very good matching, but the gain accuracy of the sys-
tem will now be dependent on the process variations of the
crosspoint resistors which have a variability of approximately
±20%.
FIGURE 1. Output Expansion
30045042
30045043
FIGURE 2. Input Expansion with Shared Termination
Resistors
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