Quad/Dual, Low-Voltage,

Bidirectional RF/Video Switches

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11

On the MAX4545, GND5 and GND6 should always be

connected to the ground plane with solid copper to

improve isolation and crosstalk.

Signal Routing

Keep all signal leads as short as possible. Separate all

signal leads from each other and other traces with the

ground plane on both sides of the board. Where possi-

ble, use coaxial cable instead of printed circuit board

traces.

Board Layout

IC sockets degrade high-frequency performance and

should not be used if signal bandwidth exceeds 5MHz.

Surface-mount parts, having shorter internal lead

frames, provide the best high-frequency performance.

Keep all bypass capacitors close to the device, and

separate all signal leads with ground planes. Such

grounds tend to be wedge-shaped as they get closer to

the device. Use vias to connect the ground planes on

each side of the board, and place the vias in the apex of

the wedge-shaped grounds that separate signal leads.

Logic-level signal lead placement is not critical.

Impedance Matching

The typical on-resistances of the switches in the

MAX4545/MAX4546/MAX4547 are 14

Ω, but the off-

state impedances are approximately equal to a 6pF

capacitor. In coaxial systems, therefore, it is impossible

to match any impedance for both the on and off state. If

impedance matching is critical, the MAX4546 is best

suited, since its two sections can be configured as a

single on/off switch, as shown in Figure 2. This circuit

“wastes” switches and has higher losses, but has bet-

ter off isolation and maintains good impedance match-

ing in both the on and off states. The resistance values

shown in Figure 3 are optimized with ±5V supplies for

both 50

Ω and 75Ω systems at room temperature.

Multiplexer

With its excellent off isolation, the MAX4545 is ideal for

use in high-frequency video multiplexers. Figure 3

shows such an application for switching any one of four

video inputs to a single output. The same circuit may

be used as a demultiplexer by simply reversing the sig-

nal direction.

Stray capacitance of traces and the output capacitance

of switches placed in parallel reduces bandwidth, so the

outputs of no more than four individual switches should

be placed in parallel if high bandwidth is to be main-

tained. If more than four mux channels are needed, the

4-channel circuit should be duplicated and cascaded.

COM1

GND1

GND1

NC4

NO1

GND4

COM4

NC3

NO2

COM2

GND2

IN2

GND3

COM3

IN1

1

3

4

6

7

8

14

15

16

12

13

11

10

9

5

V-

V-

10nF

38

Ω

(61

Ω)

38

Ω

(61

Ω)

50

Ω

OUT/IN

82

Ω

(194

Ω)

50

Ω

IN/OUT

LOGIC

IN

2

V+

V+

10nF

SWITCHES SHOWN FOR LOGIC “0” INPUT

( ) ARE FOR 75 SYSTEMS.

SWITCH

LOGIC

0

1

OFF

ON

MAX4546

Figure 2. Impedance Matching On/Off Switch