ADG749

–7–

REV. 0

APPLICATIONS INFORMATION

The ADG749 belongs to Analog Devices’ new family of

CMOS switches. This series of general purpose switches have

improved switching times, 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 important design parameters are power efficiency and

performance.

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.

Ω over the tem-

perature 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.

S

D

Figure 1. Switch Represented by Equivalent Parasitic

Components

The transfer function that describes the equivalent diagram of

the switch (Figure 1) is of the form (A)s shown below.

A(s)

+1

+1

















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. Fre-

quency plot for the ADG749 can be seen in TPC 7.

Off Isolation

Off isolation is a measure of the input signal coupled through an

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

shown in Figure 2.

S

D

Figure 2. Off Isolation Is Affected by External Load Resis-

tance and Capacitance

produced. The typical performance characteristic graph of TPC

5 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 small-

affect off isolation also, as they contribute to the coefficients of the

poles and zeros in the transfer function of the switch when open.

A(s)

=















