4 x 4

OUT

IN

1

2

3

4

1

3

4

2

4

3

2

1

1

4 x 4

OUT

IN

8

7

6

5

4

3

2

4 x 4

OUT

IN

1

2

3

4

1

3

4

2

4

3

2

1

4

3

2

1

4 x 4

OUT

IN

8

7

6

5

4 x 4

IN

OUT

4 x 4

IN

OUT

1

2

3

4

1

2

3

4

5

6

7

8

1

3

4

2

3

2

1

4

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 75

Ω 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 75

Ω 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 71

Ω. 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%.

Figure 33. Output Expansion

Figure 34. Input Expansion with

Figure 35. Input Expansion with

Shared Termination Resistors

Separate Termination Resistors

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