7

FN6099.1

December 27, 2004

Power-Supply Considerations

The ISL84780 construction is typical of most single supply

CMOS analog switches, in that they have two supply pins:

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

switches and set their analog voltage limits. Unlike switches

with a 4V maximum supply voltage, the ISL84780 4.7V

maximum supply voltage provides plenty of room for the

10% tolerance of 3.6V supplies, as well as room for

overshoot and noise spikes.

The minimum recommended supply voltage is 1.6V but the

part will operate with a supply below 1.5V. It is important to

note that the input signal range, switching times, and on-

resistance degrade at lower supply voltages. Refer to the

electrical specification tables and Typical Performance

curves for details.

V+ and GND also power the internal logic and level shifters.

The level shifters convert the input logic levels to switched

V+ and GND signals to drive the analog switch gate

terminals.

This family of switches cannot be operated with bipolar

supplies, because the input switching point becomes

negative in this configuration.

Logic-Level Thresholds

This switch family is 1.8V CMOS compatible (0.5V and 1.4V)

over a supply range of 2.0V to 3.6V (See Figure 17). At 3.6V

CMOS guaranteed high output minimum level of 1.4V, but

noise margin is reduced.

The digital input stages draw supply current whenever the

digital input voltage is not at one of the supply rails. Driving

the digital input signals from GND to V+ with a fast transition

time minimizes power dissipation.

High-Frequency Performance

In 50

Ω systems, the signal response is reasonably flat even

past 30MHz with a -3dB bandwidth of 104MHz (See

Figure 15). The frequency response is very consistent over a

wide V+ range, and for varying analog signal levels.

An OFF switch acts like a capacitor and passes higher

frequencies with less attenuation, resulting in signal

feedthrough from the switch input to its output. Off Isolation

is the resistance to this feedthrough, while Crosstalk

indicates the amount of feedthrough from one switch to

another. Figure 16 details the high Off Isolation and

Crosstalk rejection provided by this part. At 100kHz, Off

Isolation is about 68dB in 50

Ω systems, decreasing

approximately 20dB per decade as frequency increases.

Higher load impedances decrease Off Isolation and

Crosstalk rejection due to the voltage divider action of the

switch OFF impedance and the load impedance.

Leakage Considerations

Reverse ESD protection diodes are internally connected

between each analog-signal pin and both V+ and GND. One

of these diodes conducts if any analog signal 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 terminal. 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.

FIGURE 8. OVERVOLTAGE PROTECTION

GND

OPTIONAL PROTECTION

V+

DIODE

OPTIONAL PROTECTION

DIODE

OPTIONAL

PROTECTION

RESISTOR

ISL84780