REV. 0

–10–

ADG3242

Bus Isolation

A common requirement of bus architectures is low capacitance

loading of the bus. Such systems require bus bridge devices that

extend the number of loads on the bus without exceeding the

specifications. Because the ADG3242 is designed specifically for

applications that do not need drive yet require simple logic

functions, it solves this requirement. The device isolates access

to the bus, thus minimizing capacitance loading.

BUS/

BACKPLANE

LOAD A

LOAD C

LOAD B

LOAD D

BUS SWITCH

LOCATION

Figure 11. Location of Bus Switched in a Bus

Isolation Application

Hot Plug and Hot Swap Isolation

The ADG3242 is suitable for hot swap and hot plug applications.

The output signal of the ADG3242 is limited to a voltage that is

the switch acts like a buffer to take the impact from hot insertion,

protecting vital and expensive chipsets from damage.

In hot plug applications, the system cannot be shut down when

new hardware is being added. To overcome this, a bus switch can

be positioned on the backplane between the bus devices and the

hot plug connectors. The bus switch is turned off during hot plug.

Figure 12 shows a typical example of this type of application.

PLUG-IN

CARD (1)

CARD I/O

CARD I/O

RAM

CPU

PLUG-IN

CARD (2)

Figure 12. ADG3242 in a Hot Plug Application

There are many systems, such as docking stations, PCI boards

for servers, and line cards for telecommunications switches, that

require the ability to handle hot swapping. If the bus can be

isolated prior to insertion or removal, there is more control over

the hot swap event. This isolation can be achieved using bus

switches. The bus switches are positioned on the hot swap card

between the connector and the devices. During hot swap, the

ground pin of the hot swap card must connect to the ground pin

of the backplane before any other signal or power pins.

Analog Switching

Bus switches can be used in many analog switching applications,

for example, video graphics. Bus switches can have lower on

resistance, smaller ON and OFF channel capacitance, and thus

improved frequency performance than their analog counterparts.

The bus switch channel itself, consisting solely of an NMOS

switch, limits the operating voltage (see TPC 1 for a typical

plot), but in many cases, this does not present an issue.

High Impedance during Power-Up/Power-Down

To ensure the high impedance state during power-up or power-

down,

BE should be tied to V

minimum value of the resistor is determined by the current-

sinking capability of the driver.