ADG701/ADG702
Rev. C | Page 10 of 12
APPLICATIONS INFORMATION
The ADG701/ADG702 belong to the Analog Devices family
of CMOS switches. This series of general-purpose switches
has improved switching times, lower on resistance, higher
bandwidth, low power consumption, and low leakage currents.
ADG701/ADG702 SUPPLY VOLTAGES
Functionality of the ADG701/ADG702 extends from 1.8 V to
5.5 V single supply, making the parts ideal for battery-powered
instruments, where power efficiency and performance are
important design parameters.
It is important to note that the supply voltage affects the input
signal range, on resistance, and switching times of the part. The
effects of the power supplies can be clearly seen in the
Typical
Performance Characteristics
and the Specifications sections.
For V
DD
= 1.8 V operation, R
ON
is typically 40 Ω over the
temperature range.
BANDWIDTH
Figure 18 illustrates the parasitic components that affect the ac
performance of CMOS switches (a box surrounds the switch).
Additional external capacitances further degrade performance by
affecting feedthrough, crosstalk, and system bandwidth.
00039-018
V
IN
SD
R
ON
C
DS
C
D
C
LOAD
R
LOAD
V
OUT
Figure 18. Switch Represented by Equivalent Parasitic Components
The transfer function that describes the equivalent diagram of
the switch (see
Figure 18) is of the form (A)s, shown in the
following equation:
⎥
⎦
⎤
⎢
⎣
⎡
+
+
=
1)(
1)(
)(
TT
ON
DSON
T
RCRs
CRs
RsA
where C
T
= C
LOAD
+ C
D
+ C
DS
.
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. To maximize bandwidth, a
switch must have a low input and output capacitance and low
on resistance. The on response vs. frequency for the ADG701/
ADG702 can be seen in
Figure 10.
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 19.
00039-019
V
IN
SD
C
DS
C
D
C
LOAD
R
LOAD
V
OUT
Figure 19. Off Isolation Is Affected by External Load Resistance and
Capacitance
The larger the value of C
DS
, the larger the values 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 −75 dB isolation. Up to frequencies of 10 MHz,
the off isolation remains better than −55 dB. As the frequency
increases, 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))((
1)(
)(
T
LOAD
DSLOAD
T
CRs
CRs
RsA
