LMH6584VV National Semiconductor, LMH6584VV Datasheet - Page 14

• Manufacturer Part Number: LMH6584VV
• Manufacturer: National Semiconductor

Specifications of LMH6584VV

• Array Configuration: 32x16
• Number Of Arrays: 1
• Screening Level: Industrial
• Pin Count: 144
• Package Type: LQFP
• Power Supply Requirement: Dual
• Lead Free Status / RoHS Status: Supplier Unconfirmed

www.national.com
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
FIGURE 1. Output Expansion with Buffers
14
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 accomplished by connecting the cross-
point inputs and leaving the output pins on both chips sepa-
rate. The input capacitance of the crosspoint pins is 9pF when
an input is connected to one output and 12pF when an input
is connected to 16 outputs. If the crosspoint is being driven
by a 75Ω transmission line the bandwidth of the circuit will be
limited by the RC time constand of the transmission line and
the input capacitance of the two crosspoints. . In order to
eliminate this bandwidth limitation it is necessary to drive the
crosspoint inputs with a low impedance source. A circuit to
accomplish this is show in Figure 1. The circuit shown in Fig-
ure 3 will suffer severe bandwidth limitations and is not rec-
ommended.
30045007