ADG438F/ADG439F

Rev. E | Page 13 of 16

THEORY OF OPERATION

The ADG438F/ADG439F multiplexers are capable of withstanding

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 saturates,

limiting the current. The current during a fault condition is

determined by the load on the output. Figure 31 illustrates the

channel architecture that enables these multiplexers to with-

stand continuous overvoltages.

1 mA) is applied to the ADG438F/ADG439F, the multiplexer

behaves as a standard multiplexer, with specifications similar to

a standard multiplexer, for example, the on-resistance is 270 Ω

typically. However, when an overvoltage is applied to the device,

one of the three MOSFETs saturates.

Figure 29 to Figure 32 show the conditions of the three

MOSFETs for the various overvoltage situations. When the

analog input applied to an on channel approaches the positive

power supply line, the n-channel MOSFET saturates because

the voltage on the analog input exceeds the difference between

MOSFET saturates because the analog input is more negative

is used.

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

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

remains off when an overvoltage occurs.

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

is 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 over-

voltage, the first MOSFET in the series remains off because the

gate to source voltage applied to this MOSFET is negative.

During fault conditions (power supplies off), the leakage current

into and out of the ADG438F/ADG439F is limited to a few

microamps. This limit protects the multiplexer and succeeding

circuitry from over stresses as well as protects the signal sources

that drive the multiplexer. Also, the other channels of the multi-

plexer are undisturbed by the overvoltage and continue to operate

normally.

Q1

Q2

Q3

+55V

OVERVOLTAGE

n-CHANNEL

MOSFET

SATURATES

Figure 29. +55 V Overvoltage Input to the On Channel

Q1

Q2

Q3

–40V

OVERVOLTAGE

n-CHANNEL

MOSFET

IS ON

p-CHANNEL

MOSFET

SATURATES

Figure 30. −40 V Overvoltage on an Off Channel with Multiplexer Power On

Q1

Q2

Q3

+55V

OVERVOLTAGE

n-CHANNEL

MOSFET IS

OFF

Figure 31. +55 V Overvoltage with Power Off

Q1

Q2

Q3

–40V

OVERVOLTAGE

n-CHANNEL

MOSFET IS

ON

p-CHANNEL

MOSFET IS

OFF

Figure 32. –40 V Overvoltage with Power Off