9

FN7458.1

September 21, 2005

Application Circuits

Application Information

General

The ISL59421 is a 2:1 mux that is ideal as a matrix element

in high performance switchers and routers. The ISL59421 is

optimized to drive 5pF in parallel with a 500

Ω load. The

capacitance can be split between the PCB capacitance an

and external load capacitance. Its low input capacitance and

high input resistance provide excellent 50

Ω or 75Ω

terminations.

Parasitic Effects on Frequency Performance

Capacitance at the Inverting Input

The AC performance of current-feedback amplifiers in the

non-inverting gain configuration is strongly affected by stray

capacitance at the inverting input. Stray capacitance from

increase gain peaking and bandwidth. Large values of either

capacitance can cause oscillation. The ISL59421 has been

achieve optimum performance the feedback network

resistor(s) must be placed as close to the device as possible.

Trace lengths greater than 1/4 inch combined with resistor

pad capacitance can result in inverting input to ground

capacitance approaching 1pF. Inverting input and output

traces should not run parallel to each other. Small size

surface mount resistors (604 or smaller) are recommended.

Capacitance at the Output

The output amplifier is optimized for capacitance to ground

causes higher peaking with an increase in bandwidth. The

optimum range for most applications is ~1.0pF to ~6pF. The

optimum value can be achieved through a combination of

next component, and include a discrete surface mount

Feedback Resistor Values

The AC performance of the output amplifier is optimized with

recommended in the application circuits. The amplifier

bandwidth and gain peaking are directly affected by the

value(s) of the feedback resistor(s) in unity gain and gain >1

configurations. Transient response performance can be

tailored simply by changing these resistor values. Generally,

peaking. This has the effect of decreasing rise/fall times and

increasing overshoot.

Ground Connections

For the best isolation and crosstalk rejection, the GND pin

and NIC pins must connect to the GND plane.

50

Ω

357

Ω

-

+

0.6pF

PC BOARD

CAPACITANCE

3.3pF

1.6pF

50

Ω

100

Ω

-

+

100

Ω

0.6pF

PC BOARD

CAPACITANCE

4.7pF

1.6pF

ISL59421