ADG722 AD [Analog Devices], ADG722 Datasheet - Page 8

• Manufacturer Part Number: ADG722
• Description: CMOS Low Voltage 4 ohm Dual SPST Switches
• Manufacturer: AD [Analog Devices]

ADG721/ADG722/ADG723
APPLICATIONS INFORMATION
The ADG721/ADG722/ADG723 belongs to Analog Devices’
new family of CMOS switches. This series of general purpose
switches have improved switching times, lower on resistance,
higher bandwidths, low power consumption and low leakage
currents.
ADG721/ADG722/ADG723 Supply Voltages
Functionality of the ADG721/ADG722/ADG723 extends from
+1.8 V to +5.5 V single supply, which makes it ideal for battery
powered instruments, where important design parameters are
power efficiency and performance.
It is important to note that the supply voltage effects the input
signal range, the on resistance and the switching times of the
part. By taking a look at the typical performance characteristics
and the specifications, the effects of the power supplies can be
clearly seen.
For V
perature range.
On Response vs. Frequency
Figure 8 illustrates the parasitic components that affect the ac
performance of CMOS switches (the switch is shown surrounded
by a box). Additional external capacitances will further degrade
some performance. These capacitances affect feedthrough,
crosstalk and system bandwidth.
The transfer function that describes the equivalent diagram of
the switch (Figure 8) is of the form (A)s shown below.
where:
The signal transfer characteristic is dependent on the switch
channel capacitance, C
zero in the numerator of the transfer function A(s). Because the
switch on resistance is small, this zero usually occurs at high
frequencies. The bandwidth is a function of the switch output
capacitance combined with C
frequency pole corresponding to these capacitances appears in
the denominator of A(s).
The dominant effect of the output capacitance, C
pole breakpoint frequency to occur first. Therefore, in order to
maximize bandwidth a switch must have a low input and
output capacitance and low on resistance. The On Response
vs. Frequency plot for the ADG721/ADG722/ADG723 can
be seen in Figure 7.
Figure 8. Switch Represented by Equivalent Parasitic
Components
C
R
T
T
= R
= C
DD
V
IN
= +1.8 V, on resistance is typically 40
LOAD
LOAD
/(R
+ C
S
A(s)
LOAD
D
+ C
+ R
R
C
DS
DS
ON
DS
R
. This capacitance creates a frequency
T
ON
)
s(R
DS
s(R
C
ON
and the load capacitance. The
D
ON
C
D
C
T
DS
R
C
T
LOAD
) 1
) 1
R
LOAD
D
over the tem-
, causes the
V
OUT
–8–
Off Isolation
Off isolation is a measure of the input signal coupled through an
off switch to the switch output. The capacitance, C
the input signal to the output load, when the switch is off as
shown in Figure 9.
Figure 9. Off Isolation Is Affected by External Load Resis-
tance and Capacitance
The larger the value of C
produced. The typical performance characteristic graph of Fig-
ure 5 illustrates the drop in off isolation as a function of fre-
quency. From dc to roughly 1 MHz, the switch shows better
than –80 dB isolation. Up to frequencies of 10 MHz, the off
isolation remains better than –60 dB. As the frequency increases,
more and more of the input signal is coupled through to the
output. Off isolation can be maximized by choosing a switch
with the smallest C
and capacitance also affect off isolation, as they contribute to
the coefficients of the poles and zeros in the transfer function of
the switch when open.
0.006 (0.15)
0.002 (0.05)
0.122 (3.10)
0.114 (2.90)
V
SEATING
IN
PLANE
PIN 1
A(s)
0.0256 (0.65) BSC
Dimensions shown in inches and (mm).
0.122 (3.10)
0.114 (2.90)
0.120 (3.05)
0.112 (2.84)
S
8
1
0.018 (0.46)
0.008 (0.20)
OUTLINE DIMENSIONS
DS
s(R
5
4
as possible. The values of load resistance
LOAD
C
8-Lead SOIC
DS
0.199 (5.05)
0.187 (4.75)
DS
0.043 (1.09)
0.037 (0.94)
, larger values of feedthrough will be
) (C
(RM-8)
s(R
LOAD
C
LOAD
D
0.011 (0.28)
0.003 (0.08)
D
C
C
DS
C
D
LOAD
)
0.120 (3.05)
0.112 (2.84)
33°
27°
C
DS
R
) 1
LOAD
0.028 (0.71)
0.016 (0.41)
DS
V
OUT
, couples
REV. 0