ISL8394

FN6038 Rev 3.00

Page 8 of 11

March 13, 2006

1V above V-. The low leakage current performance is

unaffected by this approach, but the switch resistance may

increase, especially at low supply voltages.

Power-Supply Considerations

The ISL8394 construction is typical of most CMOS analog

switches, in that they have three supply pins: V+, V-, and

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

their analog voltage limits, so there are no connections

between the analog signal path and GND. Unlike switches

with a 13V maximum supply voltage, the ISL8394 15V

maximum supply voltage provides plenty of room for the

as well as room for overshoot and noise spikes.

This family of switches performs equally well when operated

with bipolar or single voltage supplies.The minimum

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 power the internal logic (thus setting the digital

switching point) and level shifters. The level shifters convert

the logic levels to switched V+ and V- signals to drive the

analog switch gate terminals.

Logic-Level Thresholds

V+ and GND power the internal logic stages, so V- has no

affect on logic thresholds. This switch family is TTL

compatible (0.8V and 2.4V) over a V+ supply range of 2.7V

the TTL guaranteed high output minimum level of 2.8V, but

noise margin is reduced. For best results with a 12V supply,

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 5

systems, signal response is reasonably flat even past

200MHz (see Figure 15), with a small signal -3dB bandwidth

in excess of 300MHz, and a large signal bandwidth

exceeding 300MHz.

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 16 details the high Off Isolation and Crosstalk

rejection provided by this switch. At 10MHz, off isolation is

about 50dB in 5

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 V-. One

of these diodes conducts if any analog signal exceeds V+

or V-.

Virtually all the analog leakage current comes from 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

will vary as the signal varies. The difference in the two diode

leakages to the V+ and V- 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 GND.

FIGURE 8. OVERVOLTAGE PROTECTION

V-

OPTIONAL PROTECTION

V+

DIODE

OPTIONAL PROTECTION

DIODE

OPTIONAL

PROTECTION

RESISTOR