LMH6582YA National Semiconductor, LMH6582YA Datasheet - Page 9

• Manufacturer Part Number: LMH6582YA
• Manufacturer: National Semiconductor

Specifications of LMH6582YA

• Array Configuration: 16x8
• Number Of Arrays: 1
• Screening Level: Industrial
• Pin Count: 64
• Package Type: TQFP EP
• Power Supply Requirement: Dual
• Lead Free Status / RoHS Status: Not Compliant

Application Section
INTRODUCTION
The LMH6582 is a high speed, fully buffered, non blocking,
analog crosspoint switch. Having fully buffered inputs allows
the LMH6582 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 components other than the
termination resistor. The LMH6582 can have any input con-
nected to any (or all) output(s). Conversely, a given output
can have only one associated input.
INPUT AND OUTPUT EXPANSION
The LMH6582 has high impedance inactive states for both
inputs and outputs allowing maximum flexibility for Crosspoint
expansion. In addition the LMH6582 employs diagonal sym-
metry 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 example two LMH6582
chips can be combined on one board to form either an 16 x
16 crosspoint or a 32 x 8 crosspoint. To make a 16 x 16 cross-
point all 16 input pins would be tied together (Input 0 on side
1 to input 15 on side 2 and so on) while the 8 output pins on
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each chip would be left separate. To make the 32 x 8 cross-
FIGURE 1. Output Expansion
point, the 8 outputs would be tied together while all 32 inputs
would remain independent. In the 32 x 8 configuration it is
important not to have 2 connected outputs active at the same
time. With the 16 x 16 configuration, on the other hand, having
two connected inputs active is a valid state. Crosspoint ex-
pansion as detailed above has the advantage that the signal
path has only one crosspoint in it at a time. Expansion meth-
ods that have cascaded 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. 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 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 counter act this the termination resistors of both
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crosspoints should be adjusted to approximately 71Ω. This
FIGURE 2. Input Expansion with Shared Termination
will provide very good matching, but the gain accuracy of the
Resistors
system will now be dependent on the process variations of
the crosspoint resistors which have a variability of approxi-
mately ±20%.
The LMH6582 has fully buffered inputs and outputs. The in-
puts provide a low load, high impedance input and ensure
maximum performance from a variety of signal sources. The
fully buffered outputs will drive up to two back terminated
video loads. When disabled, the outputs are in a high
impedance state. When making thermal calculations the out-
put loading conditions will be a key consideration. Please see
the section on thermal management.
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