8x4 Video Crosspoint Switches with Buffers

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13

__________Applications Information

Grounding and Bypassing,

PC Board Layout

As with all analog circuits, good PC board layout, prop-

er grounding, and careful component selection are cru-

cial for realizing the full AC performance of high-speed

amplifiers such as the MAX458/MAX459. For optimal

performance:

1) Use a large, low-impedance analog ground plane.

With multilayer boards, the ground plane(s) should

be located on the layer that does not contain signal

traces. Connect all GND pins to the analog ground

plane.

2) Minimize trace area at the circuit’s critical high-imped-

ance nodes to prevent unwanted signal coupling.

Surround analog inputs with an AC ground trace

(bypassed DC power supply, etc.). The analog input

pins of the MAX458/MAX459 have been separated

parasitic coupling, which can degrade crosstalk.

3) Connect the coaxial-cable shield to the ground side

of the 75

Ω terminating resistor at the ground plane

to further reduce crosstalk (Figure 11).

4) Bypass all power-supply pins directly to the ground

plane with 0.1µF ceramic capacitors placed as

close to the supply pins as possible. For high-cur-

rent loads, you may need 10µF tantalum or alu-

minum-electrolytic capacitors in parallel with the

0.1µF ceramics. Keep capacitor lead lengths as

short as possible to minimize series inductance; sur-

face-mount chip capacitors are ideal.

Creating Larger Arrays

The MAX458/MAX459 assume a high-impedance state

on power-up if the inputs are not being programmed to

any particular state during that time. They also are in a

high-impedance state when disabled. This feature

makes it possible to create larger arrays than 8x4 with-

out special programming, other than ensuring that your

program doesn’t turn on two paralleled outputs simulta-

neously. Testing has shown no degradation of differen-

tial gain or phase when the outputs are connected in

parallel.

The MAX458/MAX459’s input registers remain active dur-

ing shutdown, which allows the crosspoint to be pro-

grammed while the devices are shut down. As a result, all

outputs may be simultaneously brought to any state,

including disabled. Just program all of the MAX458/

MAX459s into shutdown, and enter the program of your

choice by selecting the desired inputs and outputs. Taking

SHDN low takes the device(s) out of shutdown.

A power-on reset circuit causes the output amplifiers to

power up in the disabled mode, whether or not SHDN

is applied, if

–

U

—

P

—

D

—

A

—

T

—

E

–

and

–

C

—

S

–

are high.

The number of MAX458s that can be paralleled is limit-

ed by capacitive loading on each output, which must

not exceed 100pF. Each input presents approximately

7pF of load, and each output presents approximately

12pF. Therefore, the MAX458/MAX459 will drive a maxi-

mum of 14 inputs, or 7 outputs and 2 inputs, or any

other combination resulting in less than a 100pF load.

Adding isolation resistors enables more MAX458s to be

paralleled (see the

Driving Capacitive Loads section).

MAX458

MAX459

CS

SCLK

DIN

DOUT

SCK

MOSI

I/O

MOSO

THE DOUT-MOSO CONNECTION IS NOT REQUIRED FOR

WRITING TO THE MAX458/MAX459, BUT MAY BE USED

FOR DATA-ECHO PURPOSES.

CPOL = 0, CPHA = 0

SPI/QSPI

PORT

Figure 10. SPI/QSPI Connection

RETURN

CURRENT

IN_

IN_

RETURN

CURRENT

Figure 11. Low-Crosstalk Layout. Return current from termina-

tion resistor does not flow through the ground plane.