Data Sheet

ADG749

Rev. C | Page 11 of 12

APPLICATIONS INFORMATION

The ADG749 belongs to Analog Devices’ new family of CMOS

switches. This series of general-purpose switches has improved

switching times, offering lower on resistance, higher band-

widths, 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 affects 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.

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 will

further degrade some performance. These capacitances affect

feedthrough, crosstalk, and system bandwidth.

S

D

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) shown below.

(

)

(











+

+

=

1

1

)

(

DS

ON

T

DS

ON

T

C

R

R

s

C

R

s

R

s

A

where:

The signal transfer characteristic is dependent on the switch

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

frequency pole corresponding to these capacitances appears in

the denominator of A(s).

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 ADG749 is shown in Figure 11.

OFF ISOLATION

Off isolation is a measure of the input signal coupled through

the input signal to the output load when the switch is off, as

shown in Figure 22.

S

D

Figure 22. Off Isolation is Affected by External Load Resistance

and Capacitance

The larger the value of CDS, the larger the values of feed-

through that will be produced. The typical performance

characteristic graph of Figure 9 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

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

LOAD

LOAD

DS

LOAD

C

C

C

R

s

C

R

s

s

A