8

FN6128.2

September 13, 2006

overvoltage protection for all pins (see Figure 8). These

additional diodes limit the analog signal from 1V below V+ to

1V above GND. The low leakage current performance is

unaffected by this approach, but the switch signal range is

reduced and the resistance may increase, especially at low

supply voltages.

Power-Supply Considerations

The ISL8484 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 ISL8484 4.7V

maximum supply voltage provides plenty of room for the

10% tolerance of 4.3V supplies, as well as room for

overshoot and noise spikes.

The minimum recommended supply voltage is 1.65V. It is

important to note that the input signal range, switching times,

and on-resistance degrade at lower supply voltages. Refer

to the “Electrical Specifications” tables, beginning on page 3,

and “Typical Performance Curves”, beginning on page 9, for

details.

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

The level shiftiers 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 2.7V to 4.5V (see Figure 16). At 2.7V

CMOS guaranteed low output maximum level of 0.5V, 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. The ISL8484 has been

designed to minimize the supply current whenever the digital

input voltage is not driven to the supply rails (0V to V+). For

example driving the device with 2.85V logic (0V to 2.85V)

while operating with a 4.2V supply the device draws only

High-Frequency Performance

In 50

Ω systems, the signal response is reasonably flat even

past 30MHz with a -3dB bandwidth of 120MHz (see

Figure 19). 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 a switch’s 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 20 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

ISL8484