7

FN6095.3

July 31, 2006

Detailed Description

The ISL84781 analog multiplexer offers precise switching

capability from a single 1.6V to 3.6V supply with ultra low on-

resistance (0.41

Ω) and high speed operation (t

suited for portable battery powered equipment thanks to the

low operating supply voltage (1.6V), low power consumption

(0.2

μW), and low leakage currents (70nA max). High

frequency applications also benefit from the wide bandwidth,

and the very high off isolation and crosstalk rejection.

Supply Sequencing and Overvoltage Protection

With any CMOS device, proper power supply sequencing is

required to protect the device from excessive input currents

which might permanently damage the IC. All I/O pins contain

ESD protection diodes from the pin to V+ and to GND (See

Figure 7). To prevent forward biasing these diodes, V+ must

be applied before any input signals, and the input signal

voltages must remain between V+ and GND. If these

conditions cannot be guaranteed, then one of the following

two protection methods should be employed.

Logic inputs can easily be protected by adding a 1k

Ω

resistor in series with the input (see Figure 7). The resistor

limits the input current below the threshold that produces

permanent damage, and the sub-microamp input current

produces an insignificant voltage drop during normal

operation.

This method is not applicable for the signal path inputs.

Adding a series resistor to the switch input defeats the

diodes can be added in series with the supply pins to provide

overvoltage protection for all pins (see Figure 7). 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 ISL84781 construction is typical of most single supply

CMOS analog multiplexers, in that it has two supply pins: V+

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

and set its analog voltage limits. Unlike switches with a 4V

maximum supply voltage, the ISL84781 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 power the internal logic and level shifters. The

level shifters convert the logic levels to switched V+ and

GND signals to drive the analog switch gate terminals.

These multiplexers cannot be operated with bipolar supplies,

because the input switching point becomes negative in this

configuration.

Logic-Level Thresholds

This device is 1.8V CMOS compatible (0.5V and 1.4V) over

a supply range of 2.0V to 3.6V (See Figure 12). At 3.6V the

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, signal response is reasonably flat even past

10MHz with a -3dB bandwidth of 52MHz (See Figure 16).

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 feed

through from a switch’s input to its output. Off Isolation is the

resistance to this feed through. Figure 17 details the high Off

Isolation provided by these devices. At 100kHz, Off Isolation

is about 65dB in 50

Ω 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 GND.

One of these diodes conducts if any analog signal exceeds

V+ or GND.

FIGURE 7. OVERVOLTAGE PROTECTION

GND

OPTIONAL PROTECTION

V+

LOGIC

DIODE

OPTIONAL PROTECTION

DIODE

OPTIONAL

PROTECTION

RESISTOR

FOR LOGIC

INPUTS

1k

Ω

ISL84781