Datasheet
MAX354/MAX355
Fault-Protected Analog Multiplexers
8 _______________________________________________________________________________________
_________________________________Test Circuits/Timing Diagrams (continued)
+15V
V
OUT
-15V
GND
V+
A1
V-
A0
A2
NO8
COM
MAX354
NO1
R
S
= 50Ω
V
IN
EN
10nF
R
L
1k
OFF ISOLATION = 20log
V
OUT
V
IN
10nF
+15V
V
OUT
-15V
GND
V+
A1
V-
A0
A2
NO8
COM
MAX354
NO2
R
G
= 50Ω
V
IN
EN
10nF
R
L
1k
CROSSTALK = 20log
V
OUT
V
IN
10nF
NO1
R = 1kΩ
+15V
-15V
GND
V+
A2
V-
A1
A0
NO8
MAX354
CHANNEL
SELECT
NO1
COM
EN
1MHz
CAPACITANCE
ANALYZER
f = 1MHz
Figure 5. Off Isolation Figure 6. Crosstalk
Figure 7. NO/COM Capacitance
_______________Detailed Description
Fault-Protection Circuitry
Maxim’s MAX354/MAX355 are fully fault protected for
continuous input voltages up to ±40V, whether or not
the V+ and V- power supplies are present. These
devices use a “series FET” protection scheme that not
only protects the multiplexer output from overvoltage,
but also limits the input current to sub-microamp levels.
When signal voltages exceed or are within approxi-
mately 1.5V of the supply rails, on-resistance increas-
es. This greater on-resistance limits fault currents and
output voltage, protecting sensitive circuits and com-
ponents. The protected output clamps at approximately
1.5V below the supply rails and maintains the correct
polarity. There are no glitches or polarity reversals
going into or coming out of a fault condition.
Figures 8 and 9 show how the series FET circuit protects
against overvoltage conditions. When power is off, the
gates of all three FETs are at ground. With a -25V input,
N-channel FET Q1 is turned on by the +25V gate-to-
source voltage. The P-channel device (Q2), however,
has +25V V
GS
and is turned off, thereby preventing the
input signal from reaching the output. If the input volt-
age is +25V, Q1 has a negative V
GS
, which turns it off.
Similarly, only sub-microamp leakage currents can flow
from the output back to the input, since any voltage will
turn off either Q1 or Q2.