Quad/Triple, SPDT, RGB Switches

with 250MHz Video Buffer Amplifiers

______________________________________________________________________________________

13

Another option for output disable is to connect LE to V+,

parallel the outputs of several MAX498/MAX499s, and use

E

—

N

–

to individually disable all devices but the one in use

(Figure 8b).

When the outputs are disabled, off-isolation from the

analog inputs to the amplifier outputs is typically 81dB

at 10MHz.

Grounding and Layout

The MAX498/MAX499 bandwidths are in the RF fre-

quency range. Depending on the size of the PC board

used and the frequency of operation, it may be neces-

sary to use Micro-strip or Stripline techniques.

To realize the full AC performance of these high-speed

buffers, pay careful attention to power-supply bypassing

and board layout. The PC board should have at least two

layers (wire-wrap boards are too inductive, and bread

boards are too capacitive), with one side a signal layer

and the other a large, low-impedance ground plane. With

multilayer boards, locate the ground plane on the layer

that is not dedicated to a specific signal trace. The ground

plane should be as free from voids as possible. Connect

all ground pins to the ground plane.

Connect both positive power-supply pins together and

bypass with a 0.10µF ceramic capacitor at each power-

supply pin, as close to the device as possible. Repeat

for the negative power-supply pins. The capacitor lead

lengths should be as short as possible to minimize lead

inductance; surface-mount chip capacitors are ideal. A

large-value (10µF or greater) tantalum or electrolytic

bypass capacitor on each supply may be required for

high-current loads. The location of this capacitor is not

critical.

The MAX498/MAX499’s analog input pins are isolated

with ground pins to minimize parasitic coupling, which can

degrade crosstalk and/or amplifier stability. Keep signal

paths as short as possible to minimize inductance. Ensure

that all input channel traces are the same length, to main-

tain the phase relationship between the four channels. To

further reduce crosstalk, connect the coaxial-cable shield

to the ground side of the 75

Ω terminating resistor at the

ground plane, and terminate all unused inputs to ground

and outputs with a 100

Ω or 150Ω resistor to ground.

Enables amplifier outputs. Selects

channel B.

1

0

0

Disables amplifiers. Outputs high-Z.

A0 latch = channel A.

X

1

X

Enables amplifier outputs. Latches A0

to output A or B, according to A0’s

state at C

—

S

–

’s last edge.

X

0

1

Enables amplifier outputs. Selects

channel A.

0

0

0

FUNCTION

A0

EN

CE

Latches A0,

EN. Outputs unchanged.

Enables amplifier outputs. Selects

channel B.

Enables amplifier outputs. Selects

channel A.

X

X

1

Disables amplifiers. Outputs high-Z.

X

1

0

1

0

0

0

0

0

FUNCTION

A0

EN

CS

Table 1a. Amplifier and Channel

Selection with LE = V+

Table 1b. Amplifier and Channel

Selection with LE = GND