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ADG741

CMOS Low Voltage 2 Ω SPST Switches in SC70 Packages

Analog Devices 

ADG741/ADG742

APPLICATIONS INFORMATION

The ADG741/ADG742 belong to Analog Devices’ family of

CMOS switches. This series of general-purpose switches offers

improved switching times, lower on resistance, higher

bandwidth, low power consumption, and low leakage currents.

SUPPLY VOLTAGES

Functionality of the ADG741/ADG742 extends from 1.8 V to

5.5 V single supply, which makes them ideal for battery-

powered instruments where important design parameters are

power, efficiency, and performance.

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 looking at the typical performance characteristics and

the specifications, the effects of the power supplies can be

clearly seen.

For VDD = 1.8 V operation, RON is typically 40 Ω over the

temperature range.

ON RESPONSE VS. FREQUENCY

Figure 18 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

VIN

CDS

RON

CD

D

CLOAD

RLOAD

VOUT

Figure 18. Switch Represented by Equivalent Parasitic Components

The transfer function that describes the equivalent diagram of

the switch (Figure 18) is of the form A(s), as shown below.

A(s) =

RT

⎡

⎢

⎣

s(RON ) CDS

(s RON CT RT

+1 ⎤

) + 1⎥⎦

where:

CT = C LOAD + C D + C DS

( ) RT = RLOAD RLOAD + RON

The signal transfer characteristic is dependent on the switch

channel capacitance, CDS. 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 CDS 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, CD, 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 is shown 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, CDS, couples

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

shown in Figure 19.

S

VIN

CDS

CD

D

CLOAD

RLOAD

VOUT

Figure 19. Off Isolation Affected by External

Load Resistance and Capacitance

The larger the value of CDS, the larger the value of feedthrough

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 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 and more of the input signal is

coupled through to the output. Off isolation can be maximized

by choosing a switch with the smallest CDS possible. The values

of load resistance and capacitance 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)

=

⎡

⎢

⎣

(s RLOAD C DS

(s RLOAD ) (CT )

)

+

⎤

1⎥⎦

Rev. A | Page 10 of 12

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