adg749 Analog Devices, Inc., adg749 Datasheet - Page 8

• Manufacturer Part Number: adg749
• Description: Cmos 1.8 V To 5.5 V, 2.5
  2 1 Mux/spdt Switch In Sc70 Package
• Manufacturer: Analog Devices, Inc.

ADG749
APPLICATIONS INFORMATION
The ADG749 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.
ADG749 Supply Voltages
Functionality of the ADG749 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
temperature range.
On Response vs. Frequency
Figure 1 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 1) is of the form A(s) shown below.
where:
Figure 1. Switch Represented by Equivalent Parasitic
Components
C
R
T
T
DD
=
=
C
R
V
= 1.8 V operation, R
IN
LOAD
LOAD
+
S
(
R
A(s) = R
C
LOAD
D
+
C
R
C
DS
ON
+
DS
T
R




ON
s(R
s(R
)
ON
C
T
D
ON
is typically 40 Ω over the
R
ON
D
C
DS
C
T
C
) + 1
LOAD
) + 1




R
LOAD
V
OUT
–8–
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
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 ADG749 can be seen in TPC 9.
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 2.
The larger the value of C
that will be produced. The typical performance characteristic
graph of TPC 7 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
also affect off isolation, since they contribute to the coefficients of
the poles and zeros in the transfer function of the switch when open.
Figure 2. Off Isolation Is Affected by External Load
Resistance and Capacitance
V
IN
DS
A(s) =
possible. The values of load resistance and capacitance
S




s(R
C
LOAD
DS
DS
. This capacitance creates a frequency
DS
, the larger the values of feedthrough
) (C
s(R
DS
LOAD
C
and the load capacitance. The
LOAD
D
D
+ C
C
DS
D
C
LOAD
)
+ C
DS
D
, causes the pole
R
) + 1
LOAD
DS
V
, couples
OUT




REV. A