ADG438F AD [Analog Devices], ADG438F Datasheet - Page 6

• Manufacturer Part Number: ADG438F
• Description: High Performance 4/8 Channel Fault-Protected Analog Multiplexers
• Manufacturer: AD [Analog Devices]

ADG438F/ADG439F
THEORY OF OPERATION
The ADG438F/ADG439F multiplexers are capable of with-
standing 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 sub-microamp
levels, thereby preventing the overvoltage from damaging any
circuitry following the multiplexer. Figure 12 illustrates the
channel architecture that enables these multiplexers to with-
stand continuous overvoltages.
When an analog input of V
to the ADG438F/ADG439F, the multiplexer behaves as a
standard multiplexer, with specifications similar to a standard
multiplexer, for example, the on-resistance is 180
However, when an overvoltage is applied to the device, one of
the three MOSFETs will turn off.
Figures 10 to 13 show the conditions of the three MOSFETs for
the various overvoltage situations. When the analog input ap-
plied 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
Test Circuits
Figure 11. –40 V Overvoltage on an OFF Channel with
Multiplexer Power ON
Figure 10. +55 V Overvoltage Input to the ON Channel
Test Circuit 1. On Resistance
V
S
OVERVOLTAGE
OVERVOLTAGE
S
n-CHANNEL
n-CHANNEL
MOSFET IS
MOSFET IS
R
+55V
–40V
ON
OFF
ON
I
= V
DS
V1
1
/I
DS
V
V
SS
DD
SS
+ 1.2 V to V
D
Q1
Q1
V
V
SS
DD
Q2
p-CHANNEL
Q2
MOSFET IS
DD
OFF
V
– 0.8 V is applied
S
Q3
DD
Q3
I
S
typically.
(OFF)
Test Circuit 2. I
A
V
and the
D
S2
S8
S1
V
V
–6–
DD
DD
S
n-channel threshold voltage (V
tive than V
MOSFET will turn off since the analog input is more negative
than the difference between V
voltage (V
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 remain 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
ADG438F/ADG439F is limited to a few microamps. This pro-
tects the multiplexer and succeeding circuitry from over stresses
as well as protecting the signal sources which drive the multi-
plexer. Also, the other channels of the multiplexer will be
undisturbed by the overvoltage and will continue to operate
normally.
(OFF)
V
V
SS
SS
EN
Figure 12. +55 V Overvoltage with Power OFF
Figure 13. –40 V Overvoltage with Power OFF
D
TP
OVERVOLTAGE
OVERVOLTAGE
SS
).
is applied to the multiplexer, the p-channel
+0.8V
n-CHANNEL
MOSFET IS
n-CHANNEL
MOSFET IS
+55V
–40V
OFF
ON
V
S
SS
TN
S2
S8
S1
Q1
Q1
Test Circuit 3. I
and the p-channel threshold
). When a voltage more nega-
V
V
DD
DD
p-CHANNEL
MOSFET IS
Q2
Q2
OFF
V
V
SS
SS
EN
D
D
Q3
Q3
(OFF)
+0.8V
I
D
(OFF)
A
REV. D
V
D