Power-Supply Considerations

Overview

The MAX4526/MAX4527 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 connect-

ed 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 to 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 the 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 36V

(see

Typical Operating Characteristics).

Bipolar Supplies

The MAX4526/MAX4527 operate with bipolar supplies

between ±4.5V and ±18V. However, since all factory

characterization is done with ±15V supplies, specifica-

tions at other supplies are not guaranteed. The V+ and

V- supplies need not be symmetrical, but their sum

cannot exceed the absolute maximum rating of 44V

(see

Absolute Maximum Ratings).

Phase-Reversal Analog Switches

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7

A

TIME WAVEFORMS

OUTPUT SPECTRUM

MODULATOR/DEMODULATOR CIRCUIT

LOGIC (CARRIER)

LOGIC

(CARRIER)

INPUT

OUTPUT

IN

B

A

X

Y

GND

V+

V-

V+

V+

FREQUENCY

AMPLITUDE

LOWER

SIDEBAND

UPPER

SIDEBAND

SUPPRESSED CARRIER

B

X

Y

X-Y

(OUTPUT)

MAX4526

MAX4527

Figure 2. Balanced Modulator/Demodulator