9

PS8591D

11/17/04

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PS4530/PS4531/PS4532

Precision 8-Ch, 2-Ch, Latched Analog Multiplexers/Switches

Applications Information

Power-Supply Considerations

Overview

The PS4530/PS4531/PS4532 construction is typical of most CMOS

analog Switches. They have three supply pins: V+, V, and GND. V+

and V- drive the the internal CMOS switches and set the limits of

the analog voltage on any switch. Reverse ESD-protection diodes

are internally connected between each analog-signal pin and both

V+ and V-. One of these diodes conducts if any signal exceeds V+

or V-. During normal operation, these and other reverse-biased

ESD diodes leak, forming the only current drawn from V+ or V–.

Virtually all of the analog leakage current comes from the ESD

diodes. Although the ESD diodes on a given signal pine are iden-

tical and therefore fairly well balanced, they are reverse biased

differently. Each is biased by either V+ or V- and the analog signal.

This means their leakages vary as the signal varies. The difference

in the two diode leakages to the V+ and V– pins constitutes the

analog-signal-path leakage current. All analog leakage current flows

between each pin and one of the supply terminals, not to the other

switch terminal. For this reason, both sides of a given switch can

show leakage currents of either the same or opposite polarity.

The signal paths and GND are not connected.

V+ and GND power the internal logic and logic-level translators,

and set both the input and output logic limits. The logic level

translators convert the logic levels into switched V+ and V- sig-

nals to drive the analog signals’ gates. This drive signal is the only

connection between the logic supplies and signals and the analog

supplies. V+ and V- have ESD-protection diodes to GND.

The logic-level thresholds are TTL/CMOS compatible when

V+ = +5V. As V+ rises, the threshold increases slightly, so when

V+ reaches +12V, the threshold is about 3.1V – above the TTL

guaranteed, high-level minimum of 2.8V, but still compatible with

CMOS outputs.

Bipolar Supplies

The PS4530/PS4531/PS4532 operate with bipolar supplies between

±2.0V and ±6V. The V+ and V- supplies need not be symmetrical,

but their sum cannot exceed the +13V absolute maximum rating.

Single Supply

The PS4530/PS4531/PS4532 operate from a single supply between

+2V and 12V when V- is connected to GND. All of the bipolar

precautions must be observed. At room temperature, they actually

work with a single supply at, near, or below +1.7V, although as

supply voltage decreases, switch on-resistance and switching times

become very high.

High-Frequency Performance

In 50Ohm systerns, signal response is reasonably flat up to 50MHz

(see Typical Operating Characteristics). Above 20MHz, the on

response has several minor peaks that are highly layout depen-

dent. The problem is not in turning the switch on, but in turning it

off. The off-state switch acts lilke a capacitor and passes higher

frequencies with less attenuation. At 10MHz, off isolation is about

–65dB in 50Ohm systems, becoming worse (approximately 21dB

per decade) as frequency increases. Higher circuit impedances

also make off isolation worse. Adjacent channel attenuation is

about 3dB above that of a bare IC socket, and is due entirely to

capacitive coupling.