ADG431/ADG432/ADG433
120
V
DD
= +15V
V
SS
= –15V
V
L
= +5V
100
OFF ISOLATION – dB
V
S
V
G
V
D
V
S
V
G
V
D
80
T
R
E
N
C
H
P
+
P-CHANNEL
P
+
N
–
T
R
E
N
C
H
N
+
N-CHANNEL
N
+
P
–
T
R
E
N
C
H
BURIED OXIDE LAYER
SUBSTRATE (BACKGATE)
60
Figure 1. Trench Isolation
APPLICATION
40
100
1k
10k
100k
FREQUENCY – Hz
1M
10M
TPC 7. Off Isolation vs. Frequency
Figure 2 illustrates a precise, fast sample-and-hold circuit.
An AD845 is used as the input buffer while the output opera-
tional amplifier is an AD711. During the track mode, SW1 is
closed and the output V
OUT
follows the input signal V
IN
. In
the hold mode, SW1 is opened and the signal is held by the
hold capacitor C
H
.
Due to switch and capacitor leakage, the voltage on the hold
capacitor will decrease with time. The ADG431/ADG432/
ADG433 minimizes this droop due to its low leakage specifica-
tions. The droop rate is further minimized by the use of a
polystyrene hold capacitor. The droop rate for the circuit
shown is typically 30
µV/µs.
A second switch SW2, which operates in parallel with SW1, is
included in this circuit to reduce pedestal error. Since both
switches will be at the same potential, they will have a differen-
tial effect on the op amp AD711 which will minimize charge
injection effects. Pedestal error is also reduced by the compensa-
tion network R
C
and C
C
. This compensation network also reduces
the hold time glitch while optimizing the acquisition time. Using
the illustrated op amps and component values, the pedestal error
has a maximum value of 5 mV over the
±
10 V input range. Both
the acquisition and settling times are 850 ns.
+15V
+5V
2200pF
SW2
+15V
V
IN
AD845
–15V
S
S
SW1
D
D
R
C
75
C
C
1000pF
C
H
2200pF
+15V
110
V
DD
= +15V
V
SS
= –15V
V
L
= +5V
100
CROSSTALK – dB
90
80
70
60
100
1k
10k
100k
FREQUENCY – Hz
1M
10M
TPC 8. Crosstalk vs. Frequency
TRENCH ISOLATION
In the ADG431A, ADG432A and ADG433A, an insulating
oxide layer (trench) is placed between the NMOS and PMOS
transistors of each CMOS switch. Parasitic junctions, which
occur between the transistors in junction isolated switches, are
eliminated, the result being a completely latch-up proof switch.
In junction isolation, the N and P wells of the PMOS and NMOS
transistors from a diode that is reverse-biased under normal
operation. However, during overvoltage conditions, this diode
becomes forward biased. A silicon-controlled rectifier (SCR)
type circuit is formed by the two transistors causing a significant
amplification of the current which, in turn, leads to latch up.
With trench isolation, this diode is removed, the result being a
latch-up proof switch.
AD711
V
OUT
–15V
ADG431
ADG432
ADG433
–15V
Figure 2. Fast, Accurate Sample-and-Hold
–6–
REV. C
AD [ ANALOG DEVICES ]
