High-Voltage, Fault-Protected

Analog Multiplexers

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7

Input switching, however, must be done with a fault-

protected MAX378 multiplexer, to provide the level of

protection and isolation required with most data acqui-

sition inputs. Since external signal sources may contin-

ue to supply voltage when the multiplexer and system

power are turned off, non-fault-protected multiplexers,

or even first-generation fault-protected devices, will

allow many milliamps of fault current to flow from out-

side sources into the multiplexer. This could result in

damage to either the sensors or the multiplexer. A non-

fault-protected multiplexer will also allow input overvolt-

ages to appear at its output, perhaps damaging

Sample/Holds or A/Ds. Such input overdrives may also

cause input-to-input shorts, allowing the high current

output of one sensor to possibly damage another.

The MAX378 eliminates all of the above problems. It

not only limits its output voltage to safe levels, with or

without power applied (V+ and V-), but also turns all

channels off when power is removed. This allows it to

draw only sub-microamp fault currents from the inputs,

and maintain isolation between inputs for continuous

input levels up to ±75V with power supplies off.

_______________Detailed Description

Fault Protection Circuitry

The MAX378/MAX379 are fully fault protected for contin-

uous input voltages up to ±60V, whether or not the V+

and V- power supplies are present. These devices use

a “series FET” switching scheme which not only pro-

tects the multiplexer output from overvoltage, but also

limits the input current to sub-microamp levels.

Figures 7 and 8 show how the series FET circuit pro-

tects against overvoltage conditions. When power is

off, the gates of all three FETs are at ground. With a -60V

input, N-channel FET Q1 is turned on by the +60V gate-

G

D

S

-60V

-60V

OVERVOLTAGE

N-CHANNEL MOSFET

IS TURNED ON

P-CHANNEL

MOSFET IS OFF

G

D

S

G

D

S

Figure 7. -60V Overvoltage with Multiplexer Power OFF

-15V

+15V

-15V

+13.5V

+60V

OVERVOLTAGE

N-CHANNEL MOSFET

IS TURNED ON

N-CHANNEL

MOSFET IS ON

+13.5V

OUTPUT

+15V FROM

DRIVERS

-15V FROM

DRIVERS

Figure 10. +60V Overvoltage Input to the ON Channel

-15V

+15V

-15V

-60V

-60V

OVERVOLTAGE

N-CHANNEL MOSFET

IS TURNED OFF

P-CHANNEL

MOSFET IS OFF

N-CHANNEL

MOSFET IS OFF

+60V FORCED

ON COMMON

OUTPUT

LINE BY

EXTERNAL

CIRCUITRY

-15V FROM

DRIVERS

+15V FROM

DRIVERS

Figure 9. -60V Overvoltage on an OFF Channel with

Multiplexer Power Supply ON

G

D

S

+60V

OVERVOLTAGE

N-CHANNEL MOSFET

IS TURNED OFF

G

D

S

G

D

S

Figure 8. +60V Overvoltage with Multiplexer Power OFF