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ADG508F/ADG509F/ADG528F
Rev. E | Page 11 of 20
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 submicroamp levels, thereby
preventing the overvoltage from damaging any circuitry following
the multiplexer.
Figure 17 illustrates the channel architecture that
enables these multiplexers to withstand continuous overvoltages.
When an analog input of VSS + 3 V to VDD 1.5 V is applied to
the ADG508F/ADG509F/ADG528F, the multiplexer behaves as
a standard multiplexer, with specifications similar to a standard
multiplexer, for example, the on-resistance is 400 Ω maximum.
However, when an overvoltage is applied to the device, one of
the three MOSFETs will turn off.
for the various overvoltage situations. When the analog input
applied to an ON channel approaches the positive power supply
line, the n-channel MOSFET turns OFF since the voltage on
the analog input exceeds the difference between VDD and the
n-channel threshold voltage (VTN). When a voltage more nega-
tive than VSS is applied to the multiplexer, the p-channel
MOSFET will turn off since the analog input is more negative
than the difference between VSS and the p-channel threshold
voltage (VTP). Since VTN is nominally 1.5 V and VTP is typically
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
VDD
VSS
00
03
5-
01
7
Figure 17. +55 V Overvoltage Input to the On Channel
Q1
Q2
Q3
–40V
OVERVOLTAGE
n-CHANNEL
MOSFET IS
ON
p-CHANNEL
MOSFET IS
OFF
VSS
VDD
00
03
5-
01
8
Figure 18. 40 V Overvoltage on an Off Channel with
Multiplexer Power On
Q1
Q2
Q3
+55V
OVERVOLTAGE
n-CHANNEL
MOSFET IS
OFF
00
035
-01
9
Figure 19. +55 V Overvoltage with Power Off
Q1
Q2
Q3
–40V
OVERVOLTAGE
n-CHANNEL
MOSFET IS
ON
p-CHANNEL
MOSFET IS
OFF
00
035
-02
0
Figure 20. 40 V Overvoltage with Power Off