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FN6584.0

February 25, 2009

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

The level shifters convert the input logic levels to V+ and

GND signals levels to drive the analog switch gate terminals.

A high frequency decoupling capacitor placed as close to the

V+ and GND pin as possible is recommended for proper

operation of the switch. A value of 0.1µF is highly

recommended.

Negative Signal Capability

The ISL54066 contains circuitry that allows the analog input

signal to swing below ground. The device has an analog signal

range of 6.5V below V+ up to the V+ rail (see Figure 14) while

then the analog input signal range is from -1.5V to +5V. If

V+ = 2.7V then the range is from -3.8V to +2.7V.

Logic-Level Thresholds

is still below the 1.8V CMOS guaranteed low output

maximum level of 0.45V, but noise margin is reduced. At

1.8V CMOS guaranteed high output minimum of 1.35V

under most operating conditions the switch will recognize

this as a valid logic high.

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 ISL54066 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 high while

operating with a 4.2V supply, the device draws only 1µA of

current.

High-Frequency Performance

In 50

Ω systems, the ISL54066 has a -3dB bandwidth of

30MHz (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 feed-through, 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 1MHz,

Off-Isolation is approximately 70dB in 50

Ω systems,

decreasing approximately 40dB per decade as frequency

increases. Crosstalk is approximately -80dB at 1MHz in 50

Ω

systems.

Leakage Considerations

Reverse ESD protection diodes are internally connected

between each analog-signal pin, V+ and GND. One of these

diodes conducts if any analog signal exceeds the

recommended analog signal range.

Virtually all the analog switch leakage current comes from

the ESD diodes and reversed biased junctions in the switch

cell. Although the ESD diodes on a given signal pin are

identical and therefore fairly well balanced, they are reverse

biased differently. Each is biased to either the +Ring or -Ring

and the analog input signal. This means their leakages will

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

leakages to the +Ring or -Ring and the reverse biased

junctions at the internal switch cell constitutes the

analog-signal-path leakage current.

ISL54066