ADG721/ADG722/ADG723 Data Sheet
Rev. E | Page 12 of 16
APPLICATIONS
The ADG721/ADG722/ADG723 belong to a new family of
Analog Devices CMOS switches. This series of general-purpose
switches has 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
a 1.8 V to a 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 affects the input
signal range, the on resistance, and the switching times of the part.
The typical performance characteristics and the specifications
clearly show the effects of the power supplies.
For V
DD
= 1.8 V, on resistance is typically 40 Ω over the
temperature range.
ON RESPONSE VS. FREQUENCY
Figure 21 illustrates the parasitic components that affect the ac
performance of CMOS switches (the switch is shown surrounded
by a box). Additional external capacitances further degrade some
aspects of performance. These capacitances affect feedthrough,
crosstalk, and system bandwidth.
S
D
C
D
R
LOAD
V
OUT
V
IN
C
DS
C
LOAD
R
ON
00045-021
Figure 21. Switch Represented by Equivalent Parasitic Components
The transfer function that describes the equivalent diagram of
the switch (Figure 21) is of the form (A)s, as shown in the
following equation:
( )
( )
+
+
=
1
1
)(
TT
ON
DSON
T
RCRs
CRs
RsA
where:
C
T
= C
LOAD
+ C
D
+ C
DS
R
T
= R
LOAD
/(R
LOAD
+ R
ON
)
The signal transfer characteristic is dependent on the switch
channel capacitance, C
DS
. This capacitance creates a frequency
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
DS
and the load capacitance. The
frequency pole corresponding to these capacitances appears in
the denominator of A(s).
The dominant effect of the output capacitance, C
D
, causes the
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 (see Figure 11).
OFF ISOLATION
Off isolation is a measure of the input signal coupled through
an off switch to the switch output. The capacitance, C
DS
, couples
the input signal to the output load, when the switch is off, as
shown in Figure 22.
S
D
C
D
R
LOAD
V
OUT
V
IN
C
DS
C
LOAD
00045-022
Figure 22. Off Isolation Is Affected by External Load
Resistance and Capacitance
The larger the value of C
DS
, the larger the value of feedthrough
produced. Figure 9 illustrates the drop in off isolation as a
function of frequency. 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
DS
possible. The values
of load resistance and capacitance also affect off isolation
because they contribute to the coefficients of the poles and
zeros in the transfer function of the switch when open.
( )
( )
( )
+++
=
1
)(
DS
D
LOADLOAD
DSLOAD
CCCRs
CRs
sA
