Datasheet

MAX378/MAX379
High-Voltage, Fault-Protected
Analog Multiplexers
_______________________________________________________________________________________ 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
Q
1
S
-60V
-60V
OVERVOLTAGE
N-CHANNEL MOSFET
IS TURNED ON
BECAUSE V
GS
= +60V
P-CHANNEL
MOSFET IS OFF
G
D
Q
2
S
G
D
Q
3
S
Figure 7. -60V Overvoltage with Multiplexer Power OFF
Q
1
V
TN = +1.5V
-15V +15V -15V
+13.5V
+60V
OVERVOLTAGE
N-CHANNEL MOSFET
IS TURNED ON
BECAUSE V
GS
= -45V
Q
2
Q
3
N-CHANNEL
MOSFET IS ON
+13.5V
OUTPUT
+15V FROM
DRIVERS
-15V FROM
DRIVERS
Figure 10. +60V Overvoltage Input to the ON Channel
Q
1
-15V +15V -15V
-60V
-60V
OVERVOLTAGE
N-CHANNEL MOSFET
IS TURNED OFF
BECAUSE V
GS
= +45V
Q
2
Q
3
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
Q
1
S
+60V
OVERVOLTAGE
N-CHANNEL MOSFET
IS TURNED OFF
BECAUSE V
GS
= -60V
G
D
Q
2
S
G
D
Q
3
S
Figure 8. +60V Overvoltage with Multiplexer Power OFF