Low-Voltage, Low-On-Resistance,

SPST, CMOS Analog Switches

6

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__________Applications Information

Power-Supply Considerations

The MAX4514/MAX4515 construction is typical of most

CMOS analog switches, except that they have only two

supply pins: V+ and GND. V+ and GND drive the inter-

nal CMOS switches and set their analog voltage limits.

Reverse ESD-protection diodes are internally connect-

ed between each analog-signal pin and both V+ and

GND. One of these diodes conducts if any analog sig-

nal exceeds V+ or GND.

Virtually all the analog leakage current comes from the

ESD diodes to V+ or GND. 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 GND and the analog signal. This

means their leakages will vary as the signal varies. The

difference in the two diode leakages to the V+ and GND

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 termi-

nal. This is why both sides of a given switch can show

leakage currents of the same or opposite polarity.

There is no connection between the analog-signal

paths and V+ or GND.

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

level translators. The logic-level translators convert the

logic levels to switched V+ and GND signals to drive

the analog signal gates.

Logic-Level Thresholds

The logic-level thresholds are CMOS/TTL compatible

when V+ is +5V. As V+ is raised, the level threshold

increases slightly. When V+ reaches +12V, the level

threshold is about 3.0V—above the TTL guaranteed

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

CMOS outputs.

Do not connect the MAX4514/MAX4515’s V+ to +3V

and then connect the logic-level pins to logic-level

signals that operate from +5V supply. Output levels

can exceed +3V and violate the absolute maximum

ratings, damaging the part and/or external circuits.

High-Frequency Performance

In 50

Ω systems, signal response is reasonably flat up to

250MHz (see

Typical Operating Characteristics). Above

20MHz, the on response has several minor peaks that

are highly layout dependent. The problem is not in turn-

ing the switch on; it’s in turning it off. The off-state switch

acts like a capacitor and passes higher frequencies with

less attenuation. At 10MHz, off isolation is about -45dB

in 50

Ω systems, decreasing (approximately 20dB per

decade) as frequency increases. Higher circuit imped-

ances also make off isolation decrease. Off isolation is

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

entirely to capacitive coupling.

V+

0V

MAX4515

MAX4514

ERROR Q WHEN THE CHANNEL TURNS OFF.

Q =

V+

GND

V+

IN

NO

or

NC

COM

50

Ω

MAX4514

MAX4515

1000pF

Figure 1. Charge Injection

______________________________________________Test Circuits/Timing Diagrams