6

unaffected by this approach, but the switch resistance may

increase, especially at low supply voltage.

Power-Supply Considerations

The ISL8451X construction is typical of most CMOS analog

switches, except that there are only two supply pins: V+ and

V-. The power supplies need not be symmetrical for useful

operation. As long as the total supply voltage (V+ to V-,

including supply tolerances, overshoot, and noise spikes) is

less than the15V maximum supply rating, and the digital

input switching point remains reasonable (see “Logic-Level

Thresholds” section), the ISL84516, ISL84517 function well.

The 15V maximum supply rating provides the designer of

12V systems much greater flexibility than switches with a

13V maximum supply voltage.

The minimum recommended supply voltage is

±1.5V. It is

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

and on-resistance degrade at lower supply voltages, and the

“Typical Performance” curves for details.

V+ and V- power the internal CMOS switches and set their

analog voltage limits. These supplies also power the internal

logic and level shifters. The level shifters convert the input

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

switch gate terminals.

This family of switches is not recommended for single supply

applications. For single supply, similar performance, pin

compatible, TTL compatible versions of these switches, see

the ISL84514, ISL84515 data sheet.

Logic-Level Thresholds

Due to the lack of a GND pin, the switching point of the

digital input is referenced predominantly to V+. The digital

input is CMOS compatible at

±5V supplies, and is TTL

compatible for

±3.3V supplies. For other supply

combinations refer to Figure 11.

The switching point has a very low temperature sensitivity,

of supply voltage.

High-Frequency Performance

In 5

0Ω systems, signal response is reasonably flat to

30MHz, with a -3dB bandwidth of nearly 400MHz (see

Figure 13). Figure 13 also illustrates that 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. Figure 14 details the

high OFF Isolation provided by this family. At 10MHz, OFF

Isolation is about 50dB in 5

0Ω systems, decreasing

approximately 20dB per decade as frequency increases.

Higher load impedances decrease OFF Isolation 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 V+ or V-.

V-

OPTIONAL PROTECTION

V+

IN

DIODE

OPTIONAL PROTECTION

DIODE

OPTIONAL

PROTECTION

RESISTOR

FIGURE 6. OVERVOLTAGE PROTECTION

ISL84516, ISL84517