ADG438F/ADG439F

–6–

REV. D

MOSFET will turn off since the analog input is more negative

When the power supplies are present but the channel is off,

again either the p-channel MOSFET or one of the n-channel

MOSFETs will remain off when an overvoltage occurs.

Finally, when the power supplies are off, the gate of each

MOSFET will be at ground. A negative overvoltage switches on

the first n-channel MOSFET but the bias produced by the

overvoltage causes the p-channel MOSFET to remain turned

off. With a positive overvoltage, the first MOSFET in the series

will remain off since the gate to source voltage applied to this

MOSFET is negative.

During fault conditions, the leakage current into and out of the

ADG438F/ADG439F is limited to a few microamps. This pro-

tects the multiplexer and succeeding circuitry from over stresses

as well as protecting the signal sources which drive the multi-

plexer. Also, the other channels of the multiplexer will be

undisturbed by the overvoltage and will continue to operate

normally.

Q1

Q2

Q3

+55V

OVERVOLTAGE

n-CHANNEL

MOSFET IS

OFF

Figure 12. +55 V Overvoltage with Power OFF

Q1

Q2

Q3

–40V

OVERVOLTAGE

n-CHANNEL

MOSFET IS

ON

p-CHANNEL

MOSFET IS

OFF

Figure 13. –40 V Overvoltage with Power OFF

THEORY OF OPERATION

The ADG438F/ADG439F multiplexers are capable of with-

standing overvoltages from –40 V to +55 V, irrespective of

whether the power supplies are present or not. Each channel of

the multiplexer consists of an n-channel MOSFET, a p-channel

MOSFET and an n-channel MOSFET, connected in series.

When the analog input exceeds the power supplies, one of the

MOSFETs will switch off, limiting the current to sub-microamp

levels, thereby preventing the overvoltage from damaging any

circuitry following the multiplexer. Figure 12 illustrates the

channel architecture that enables these multiplexers to with-

stand continuous overvoltages.

to the ADG438F/ADG439F, the multiplexer behaves as a

standard multiplexer, with specifications similar to a standard

multiplexer, for example, the on-resistance is 180

Ω typically.

However, when an overvoltage is applied to the device, one of

the three MOSFETs will turn off.

Figures 10 to 13 show the conditions of the three MOSFETs for

the various overvoltage situations. When the analog input ap-

plied to an ON channel approaches the positive power supply

line, the n-channel MOSFET turns OFF since the voltage on

Q1

Q2

Q3

+55V

OVERVOLTAGE

n-CHANNEL

MOSFET IS

OFF

Figure 10. +55 V Overvoltage Input to the ON Channel

Q1

Q2

Q3

–40V

OVERVOLTAGE

n-CHANNEL

MOSFET IS

ON

p-CHANNEL

MOSFET IS

OFF

Figure 11. –40 V Overvoltage on an OFF Channel with

Multiplexer Power ON

Test Circuits

S

V1

D

Test Circuit 1. On Resistance

S1

S2

S8

+0.8V

D

EN

A

S1

S2

S8

+0.8V

D

EN

A