2.5

CMOS Analog Switches

8

_______________________________________________________________________________________

Applications Information

Overvoltage Protection

Proper power-supply sequencing is recommended for

all CMOS devices. Do not exceed the absolute maxi-

mum ratings, because stresses beyond the listed rat-

ings can cause permanent damage to the devices.

Always sequence V+ on first, then V-, followed by the

logic inputs, NO, or COM. If power-supply sequencing

is not possible, add two small signal diodes (D1, D2) in

series with supply pins for overvoltage protection

(Figure 1). Adding diodes reduces the analog signal

range to one diode drop below V+ and one diode drop

above V-, but does not affect the devices’ low switch

resistance and low leakage characteristics. Device

operation is unchanged, and the difference between

V+ and V- should not exceed 44V. These protection

diodes are not recommended when using a single supply.

Off-Isolation at High Frequencies

With the N.C. pins connected to GND, the high-fre-

quency on-response of these parts extends from DC to

above 100MHz with a typical loss of -2dB. When the

switch is turned off, however, it behaves like a capaci-

tor, and off-isolation decreases with increasing frequen-

cy. (Above 300MHz, the switch actually passes more

signal turned off than turned on.) This effect is more

pronounced with higher source and load impedances.

Above 5MHz, circuit-board layout becomes critical, and

it becomes difficult to characterize the response of the

switch independent of the circuit. The graphs shown in

the

Typical Operating Characteristics were taken using

a 50

Ω source and load connected with BNC connectors

to a circuit board deemed “average;” that is, designed

with isolation in mind, but not using strip-line or other

special RF circuit techniques. For critical applications

above 5MHz, use the MAX440, MAX441, and MAX442,

which are fully characterized up to 160MHz.

COM_

V-

V+

NO_

* INTERNAL PROTECTION DIODES

D2

D1

-15V

+15V

MAX4607

MAX4608

MAX4609

*

*

*

*

LOGIC

INPUT

V-

-15V

100

Ω

NO_

OR NC_

GND

(

)

SWITCH

INPUT

IN_

COM_

SWITCH

OUTPUT

tr < 20ns

tf < 20ns

50%

0V

+3V

0V

SWITCH

OUTPUT

LOGIC

INPUT

LOGIC INPUT WAVEFORMS INVERTED FOR SWITCHES

THAT HAVE THE OPPOSITE LOGIC SENSE.

VL

V+

35pF

+5V

+15V

0

REPEAT TEST FOR EACH SWITCH. FOR LOAD

CONDITIONS, SEE

ELECTRICAL CHARACTERISTICS.

MAX4607

MAX4608

MAX4609

Figure 1. Overvoltage Protection Using External Blocking

Diodes

Figure 2. Switching-Time Test Circuit

Test Circuits/Timing Diagrams