LMH6583_14 Datasheet, PDF (11/24 Pages) Texas Instruments – LMH6583 16x8 550 MHz Analog Crosspoint Switch, Gain of 2

LMH6583
www.ti.com
SNOSAP5E – APRIL 2006 – REVISED MARCH 2013
APPLICATION INFORMATION
INTRODUCTION
The LMH6583 is a high speed, fully buffered, non blocking, analog crosspoint switch. Having fully buffered inputs
allows the LMH6583 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. When disabled, the outputs are in a high impedance
state. The LMH6583 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 LMH6583 has high impedance inactive states for both inputs and outputs allowing maximum flexibility for
Crosspoint expansion. In addition the LMH6583 employs 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 example two LMH6583 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 each chip would be left separate. To make the 32 x 8 crosspoint,
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 expansion as detailed above has the
advantage that the signal path has only one crosspoint in it at a time. Expansion methods 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 inputs of two crosspoint switches has a very minor
impact on performance. Input expansion requires more planning. As shown in Figure 34 and Figure 35 there are
two ways to connect the outputs of the crosspoint switches. In Figure 34 the crosspoint switch outputs are
connected directly together and share one termination resistor. This is the easiest configuration 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 peaking.
This is illustrated in Figure 36 and Figure 37. In most cases this small amount of peaking is not a problem.
As illustrated in Figure 35 each crosspoint output can be given its own termination resistor. This results in a
frequency response 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 71Ω. 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 approximately
±20%.
1
1
1
2
2
IN
3
4 x 4 OUT
3
4
4
2
3
1
5
4
2
6
3 ^IN
4 x 4 _{OUT 7}
4
8
1
1
1
2
2
IN
4x4
OUT
3
3
2
4
4
3
5
1
4
6
IN
7
4x4
2
OUT
3
1
1
1
2
2
IN
4x4
OUT
3
3
2
4
4
3
5
1
4
6
IN
7
4x4
2
OUT
3
8
4
8
4
Figure 33. Output Expansion
Figure 34. Input Expansion with
Shared Termination Resistors
Copyright © 2006–2013, Texas Instruments Incorporated
Product Folder Links: LMH6583
Figure 35. Input Expansion with
Separate Termination Resistors
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