ADG508F/ADG509F/ADG528F

REV. C

–7–

THEORY OF OPERATION

The ADG508F/ADG509F/ADG528F 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 will switch off, limiting the current to sub-

microamp levels, thereby preventing the overvoltage from dam-

aging any circuitry following the multiplexer. Figure 12 illustrates

the channel architecture that enables these multiplexers to with-

stand continuous overvoltages.

the ADG508F/ADG509F/ADG528F, the multiplexer behaves

as a standard multiplexer, with specifications similar to a stan-

dard multiplexer, for example, the on-resistance is 400

Ω maxi-

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

one of the three MOSFETs will turn off.

Figures 12 to 15 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 12. +55 V Overvoltage Input to the ON Channel

Q1

Q2

Q3

–40V

OVERVOLTAGE

n-CHANNEL

MOSFET IS

ON

p-CHANNEL

MOSFET IS

OFF

Figure 13. –40 V Overvoltage on an OFF Channel with

Multiplexer Power ON

MOSFET will turn off since the analog input is more negative

3 V, the analog input range to the multiplexer is limited to

–12 V to +13.5 V when a

±15 V power supply 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 will turn 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

ADG508F/ADG509F/ADG528F is limited to a few microamps.

This protects the multiplexer and succeeding circuitry from over

stresses as well as protecting the signal sources which drive the

multiplexer. 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 14. +55 V Overvoltage with Power OFF

Q1

Q2

Q3

–40V

OVERVOLTAGE

n-CHANNEL

MOSFET IS

ON

p-CHANNEL

MOSFET IS

OFF

Figure 15. –40 V Overvoltage with Power OFF