8

FN6088.5

July 31, 2007

High-Frequency Performance

In 50

Ω systems, the ISL84684 has 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 62dB

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.

ISL84684