LMH6580, LMH6581

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SNOSAY4C – AUGUST 2007 – REVISED MAY 2013

APPLICATION INFORMATION

INTRODUCTION

The LMH6580/LMH6581 are high speed, fully buffered, non- blocking, analog crosspoint switches. Having fully

buffered inputs allows the LMH6580/LMH6581 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 LMH6580/LMH6581 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 LMH6580/LMH6581 have high impedance inactive states for both inputs and outputs allowing maximum

flexibility for crosspoint expansion. In addition the LMH6580/LMH6581 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 example two LMH6580/LMH6581 chips can be combined on one board to form

either an 8 x 8 crosspoint or a 16 x 4 crosspoint. To make an 8 x 8 crosspoint all 8 input pins would be tied

together (Input 0 on side 1 to input 7 on side 2 and so on) while the 4 output pins on each chip would be left

separate. To make the 16 x 4 crosspoint, the 4 outputs would be tied together while all 16 inputs would remain

independent. In the 16 x 4 configuration it is important not to have 2 connected outputs active at the same time.

With the 8 x 8 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 will go through only one crosspoint. Expansion

methods that have cascaded stages will suffer bandwidth loss far greater than the small loading effect of parallel

expansion.

Output expansion as shown in Figure 59 is very straight forward. Connecting the inputs of two crosspoint

switches has a very minor impact on performance. Input expansion requires more planning. Input expansion, as

show in Figure 60 and Figure 61 gives the option of two ways to connect the outputs of the crosspoint switches.

In Figure 60 the crosspoint switch outputs are connected directly together and share one termination resistor.

This is the easiest configurarion 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 62 and Figure 63. In most cases

this small amount of peaking is not a problem.

As illustrated in Figure 61 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 80

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%.

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Product Folder Links: LMH6580 LMH6581