ADG719
Rev. D | Page 12 of 16
APPLICATIONS INFORMATION
The ADG719 belongs to Analog Devices’ new family of CMOS
switches. This series of general-purpose switches has improved
switching times, lower on resistance, higher bandwidths, low
power consumption, and low leakage currents.
ADG719 SUPPLY VOLTAGES
Functionality of the ADG719 extends from 1.8 V to 5.5 V single
supply, which makes it ideal for battery-powered instruments
where power efficiency and performance are important design
parameters.
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
DD
= 1.8 V operation, R
ON
is typically 40 Ω over the
temperature range.
ON RESPONSE VS. FREQUENCY
Figure 22 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.
S
C
D
D
V
OUT
R
LOAD
C
LOAD
IN
R
ON
C
DS
08708-022
Figure 22. Switch Represented by Equivalent Parasitic Components
The transfer function that describes the equivalent diagram of
the switch (Figure 22) is of the form A(s) shown below:
⎥
⎥
⎦
⎤
⎢
⎢
⎣
⎡
+
+
=
1)(
1)(
)(
T
ON
T
DSON
T
CRRs
CRs
RsA
where:
R
T
= R
LOAD
/(R
LOAD
+ R
ON
)
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. 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 ADG719 can be seen in Figure 12.
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 23.
S
C
D
D
V
OUT
R
LOAD
C
LOAD
IN
C
DS
08708-023
Figure 23. Off Isolation Is Affected by External Load Resistance and
Capacitance
The larger the value of C
DS
, the larger the values of feedthrough
that will be produced. Figure 10 illustrates the drop in off
isolation as a function of frequency. From dc to roughly 200 kHz,
the switch shows better than −95 dB isolation. Up to frequencies of
10 MHz, the off isolation remains better than −67 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, since 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
