參數(shù)資料
型號: ADG508F
廠商: Analog Devices, Inc.
英文描述: 4/8 Channel Fault-Protected Analog Multiplexers
中文描述: 4 / 8通道故障保護(hù)模擬多路復(fù)用器
文件頁數(shù): 7/12頁
文件大?。?/td> 170K
代理商: ADG508F
ADG508F/ADG509F/ADG528F
REV. C
–7–
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 sub-
microamp levels, thereby preventing the overvoltage from dam-
aging 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
SS
+ 3 V to V
DD
– 1.5 V is applied to
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
maxi-
mum. However, when an overvoltage is applied to the device,
one of the three MOSFETs will turn off.
Figures 12 to 15 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
DD
and the
Q1
Q2
Q3
+55V
OVERVOLTAGE
n-CHANNEL
MOSFET IS
OFF
V
DD
V
SS
Figure 12. +55 V Overvoltage Input to the ON Channel
Q1
Q2
Q3
–40V
OVERVOLTAGE
n-CHANNEL
MOSFET IS
ON
V
DD
V
SS
p-CHANNEL
MOSFET IS
OFF
Figure 13. –40 V Overvoltage on an OFF Channel with
Multiplexer Power ON
n-channel threshold voltage (V
TN
). When a voltage more nega-
tive than V
SS
is applied to the multiplexer, the p-channel
MOSFET will turn off since the analog input is more negative
than the difference between V
SS
and the p-channel threshold
voltage (V
TP
). Since V
TN
is nominally 1.5 V and V
TP
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
Figure 14. +55 V Overvoltage with Power OFF
Q1
Q2
Q3
–40V
OVERVOLTAGE
n-CHANNEL
MOSFET IS
ON
p-CHANNEL
MOSFET IS
OFF
Figure 15. –40 V Overvoltage with Power OFF
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