adg509f Analog Devices, Inc., adg509f Datasheet - Page 7

• Manufacturer Part Number: adg509f
• Description: 4/8 Channel Fault-protected Analog Multiplexers
• Manufacturer: Analog Devices, Inc.

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 3 illustrates the channel
architecture that enables these multiplexers to withstand
continuous overvoltages.
When an analog input of V
the ADG508F/ADG509F/ADG528F, the multiplexer behaves
as a standard multiplexer, with specifications similar to a stan-
dard 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.
Figures 3 to 6 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 turns OFF since the voltage on the analog
input exceeds the difference between V
OVERVOLTAGE
OVERVOLTAGE
n-CHANNEL
n-CHANNEL
MOSFET IS
MOSFET IS
+55V
–40V
OFF
ON
V
V
SS
DD
SS
+ 3 V to V
Q1
Q1
V
V
SS
DD
DD
DD
p-CHANNEL
Q2
Q2
MOSFET IS
– 1.5 V is applied to
OFF
and the n-channel
Q3
Q3
threshold voltage (V
is applied to the multiplexer, the p-channel MOSFET will turn
off since the analog input is more negative than the difference
between V
V
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 multi-
plexer will be undisturbed by the overvoltage and will continue
to operate normally.
TN
is nominally 1.5 V and V
SS
OVERVOLTAGE
OVERVOLTAGE
and the p-channel threshold voltage (V
ADG508F/ADG509F/ADG528F
n-CHANNEL
n-CHANNEL
MOSFET IS
MOSFET IS
+55V
–40V
TN
OFF
ON
). When a voltage more negative than V
TP
Q1
Q1
is typically 3 V, the analog
p-CHANNEL
MOSFET IS
Q2
Q2
OFF
Q3
Q3
TP
). Since
SS