Low-Voltage, Phase-Reversal

Analog Switch

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7

_______________Detailed Description

The MAX4528 is a phase-reversal analog switch consist-

ing of two normally open and two normally closed CMOS

analog switches arranged in a bridge configuration.

Analog signals are put into two input pins and taken out

of two output pins. A logic-level signal controls whether

the input signal is routed through normally or inverted. A

low-resistance DC path goes from inputs to outputs at all

times, yet isolation between the two signal paths is excel-

lent. Analog signals range from V- to V+.

These parts are characterized and optimized with ±5V

supplies, and can operate from a single supply.

The MAX4528 is designed for DC and low-frequency-

signal phase-reversal applications, such as chopper

amplifiers, modulator/demodulators, and self-zeroing or

self-calibrating circuits. Unlike conventional CMOS

switches externally wired in a bridge configuration,

both DC and AC symmetry are optimized with a small

8-pin configuration that allows simple board layout and

isolation of logic signals from analog signals.

Power-Supply Considerations

Overview

The MAX4528’s construction is typical of most CMOS

analog switches. It has three supply pins: V+, V-, and

GND. V+ and V- drive the internal CMOS switches and

set the analog-voltage limits 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 analog signal

exceeds V+ or V-.

Virtually all of the analog leakage current is through the

ESD diodes to V+ or V-. Although the ESD diodes on a

given signal pin are identical 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

dif-

ference in the two diode leakages from the signal path

to the V+ and V- pins constitutes the analog-signal-path

leakage current. All analog leakage current flows to the

supply terminals, not to the other switch terminal. This

explains how both sides of a given switch can show

leakage currents of either the same or opposite polarity.

There is no connection between the analog-signal

paths and GND. The analog-signal paths consist of an

N-channel and P-channel MOSFET with their sources

and drains paralleled and their gates driven out-of-

phase to V+ and V- by the logic-level translators.

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

translator and set the input logic threshold. The logic-

level translator converts the logic levels to switched V+

and V- signals to drive the analog switches’ gates. This

drive signal is the only connection between GND and

the analog supplies. V+ and V- have ESD-protection

diodes to GND. The logic-level input has ESD protec-

tion to V+ and V-, but not to GND, so the logic signal

can go below GND (as low as V-) when bipolar sup-

plies are used.

Increasing V- has no effect on the logic-level thresholds,

but it does increase the drive to the internal P-channel

switches, reducing overall switch on-resistance. V- also

sets the negative limit of the analog-signal voltage.

The logic-level input pin (IN) has ESD-protection diodes

to V+ and V- but not to GND, so it can be safely driven

TTL compatible when V+ is between 4.5V and 12V

(see

Typical Operating Characteristics).

NAME

FUNCTION

5

V-

Negative Analog Supply-Voltage

Input. Connect V- to GND for single-

supply operation.

PIN

1

A

Analog-Switch Input Terminal A.

Connected to Y when IN is low; con-

nected to X when IN is high.

6

Y

Analog-Switch Output Terminal Y

7

X

Analog-Switch Output Terminal X

2

B

Analog-Switch Input Terminal B.

Connected to X when IN is low; con-

nected to Y when IN is high.

8

V+

Positive Analog/Digital Supply-Voltage

Input. Internally connected to sub-

strate.

4

IN

Logic-Level Control Inputs (see

Truth

Table)

3

GND

Ground. Connect GND to digital

ground. (Analog signals have no

ground reference; they are limited to

V+ and V-.)

Note:

Pins A, B, X, and Y are identical and interchangeable.

Any may be considered as an input or output; signals pass

equally well in either direction. However, AC symmetry is best

when A and B are the inputs and X and Y are the outputs.

Reduce AC balance in critical applications by using A and X or

A and Y as the input, and B and X or B and Y as the output.

_____________________Pin Description