ST AN1725 APPLICATION NOTE

AN1725

APPLICATION NOTE

ADVANCED PERFORMANCE USING STG3699

M. Porto

1. ABSTRACT

IC analog switches t raditionally have found l imited use in applications involving high - freque ncy analog
or digital signals.
Degradation of switch performance and intolerable signal cross-talk between channels have undoubtedly
forced many microelectronics manu facturers to study new technologie s to solve the problem.

2. DEVICE DESCRIPTION

The STG3699 is a high speed CMOS with low on resistance and low voltage quad analog S.P.D.T.
(Single Pole Dual Throw) switch or quad 2:1 Multiplexer / Demultiplexer switch fabricat ed in C2MOS
technology.
These Analog Switches are designed to ope rate from 1.65V to 3.6V and offer asymmetrical normally
closed (NC) and normally open (NO) R

for applications that require asymmetrical loads (such as

ON

speaker headsets and internal speakers).
The STG369 9 has Brea k-Before-Make s witching and f ast swit ching features (t

= 2.7V).

at V

cc

ON

= t

= 30ns Typical

OFF

Figure 1: Quad Analog Switch

June 2003

1-2IN

3-4IN

1S1
1S2

2S1

2S2

3S1

3S2

4S1
4S2

VCC

GND

COM

COM

COM

COM

D1

D2

D3

D4

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AN1725 - APPLICATION NOTE

3. CROSS-TALK

The main sources of cross-talk are caused by parasitic line inductance generated by transmission line of
PCB and parasitic die capacitance generated by internal structure of the die.
The single channel cross-talk can be defined as the ratio between the unwanted signal present in a
channel switched ON, and the signal applied to another single Off channel input.
Figure2 shows the single channel cross-talk phenomenon.

Figure 2: Cross-Talk Phenomenon

Parasitic Line Inductance

Input Signal

Unwante d Signal

Channel OFF

Channel ON

Parasitic Die Capacitance

The value of cross talk are expressed in decibel by the following formula:
XTALK (dB) = 20log V

is the value of the unwanted signal present in the channel switche d ON, while Vin is the value of the

V

out

out

/ Vin

signal applied to the other single channel OFF input.
The internal architecture of the STG3699 device allows very low values of unwanted signal thus avoiding
interference between different signals applied on switch.

Figure 3: Cross-Talk

Cross - Tal k

Cross - Tal k

dB

dB

1.5 1.8 2.1 2.4 2.7 3 3.3 3.6

1.5 1.8 2.1 2.4 2.7 3 3.3 3.6

-20

-20

-25

-25

-30

-30

-35

-35

-40

-40

-45

-45

-50

-50

-55

-55

-60

-60

-65

-65
Vcc (V)

Vcc (V)

SSS Avg

SSS Avg

Temp. = 25°C

Temp. = 25°C
Vcc = from 1.65V to 3.6V

Vcc = from 1.65V to 3.6V
Corner = SSS

Corner = SSS

The previous chart shows the values (expressed in dB) of cross-talk depending on Power Supply.

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