REV. 0

–10–

ADG3249

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.

Multiplexing

Many systems, such as docking stations and memory banks,

have a large number of common bus signals. Common prob-

lems faced by designers of these systems include

• Large delays caused by capacitive loading of the bus

• Noise due to simultaneous switching of the address and data

bus signals

Figure 11 shows an array of memory banks in which each ad-

dress and data signal is loaded by the sum of the individual

loads. If a bus switch is used as shown in Figure 12, the output

load on the memory address and data bits is halved. The speed

at which the selected bank’s data can flow is much improved

because the capacitance loading is halved and the switches

introduce negligible propagation delay. Bus noise is also reduced.

High Impedance during Power-Up/Power-Down

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

down,

EN should be tied to V

minimum value of the resistor is determined by the current-

sinking capability of the driver.

MEMORY

ADDRESS

DATA

MEMORY

BANK B

MEMORY

BANK C

MEMORY

BANK D

MEMORY

BANK A

Figure 11. All Memory Banks Are Permanently

Connected to the Bus

MEMORY

ADDRESS

DATA

MEMORY

BANK B

MEMORY

BANK C

MEMORY

BANK D

MEMORY

BANK A

Figure 12. ADG3249 Used to Reduce Both Access

Time and Noise