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.
When an analog input of V
SS
+ 2.2 V to V
DD
– 2.2 V (output loaded,
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
V
DD
and the n-channel threshold voltage (V
TN
). When a voltage
more negative than V
SS
is applied to the multiplexer, the p-channel
MOSFET saturates because the analog input is more negative
than the difference between V
SS
and the p-channel threshold
voltage (V
TP
). Because V
TN
is nominally 1.4 V and V
TP
− 1.4 V,
the analog input range to the multiplexer is limited to V
SS
+ 1.4 V
to V
DD
− 1.4 V (output open circuit) 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
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
V
DD
V
SS
00468-017
Figure 29. +55 V Overvoltage Input to the On Channel
Q1 Q2 Q3
–40V
OVERVOLTAGE
n-CHANNEL
MOSFET
IS ON
p-CHANNEL
MOSFET
SATURATES
V
SS
V
DD
00468-018
Figure 30. −40 V Overvoltage on an Off Channel with Multiplexer Power On
Q1 Q2 Q3
+55V
OVERVOLTAGE
n-CHANNEL
MOSFET IS
OFF
00468-019
Figure 31. +55 V Overvoltage with Power Off
Q1 Q2 Q3
–40V
OVERVOLTAG E
n-CHANNEL
MOSFET IS
ON
p-CHANNEL
MOSFET IS
OFF
00468-020
Figure 32. –40 V Overvoltage with Power Off
