ISL59421 Intersil Corporation, ISL59421 Datasheet - Page 9

• Manufacturer Part Number: ISL59421
• Description: 865MHz Multiplexing Amplifier
• Manufacturer: Intersil Corporation

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 500Ω 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 50Ω 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
the inverting input pin to the output (C
increase gain peaking and bandwidth. Large values of either
capacitance can cause oscillation. The ISL59421 has been
optimized for a 0.4pF to 0.7pF capacitance (C
Capacitance (C
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.
F
) to the output should be minimized. To
V
IN
V
IN
9
50Ω
50Ω
F
), and to ground (C
0.4pF < C
0.6pF
FIGURE 25A.
0.4pF < C
FIGURE 25B.
100Ω
0.6pF
G
).
G
PC BOARD
CAPACITANCE
< 0.7pF
C
G
357Ω
+
-
G
PC BOARD
CAPACITANCE
< 0.7pF
C
100Ω
+
-
G
G
GAIN OF 1 APPLICATION CIRCUIT
GAIN OF 2 APPLICATION CIRCUIT
ISL59421
),
*C
1.6pF
L
C
*C
Capacitance at the Output
The output amplifier is optimized for capacitance to ground
(C
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
PC board trace capacitance (C
(C
pin capacitance is to minimize the trace length (C
next component, and include a discrete surface mount
capacitor (C
Feedback Resistor Values
The AC performance of the output amplifier is optimized with
the feedback resistor network (R
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,
lower values of R
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.
= C
T
1.6pF
L
C
L
OUT)
= C
T
T
) directly on the output pin. Increased capacitance
+ C
T
+ C
. A good method to maintain control over the output
OUT
*C
C
C
L
T
OUT
OUT
: TRACE CAPACITANCE
: TOTAL LOAD CAPACITANCE
C
3.3pF
OUT
OUT
: OUTPUT CAPACITANCE
C
4.7pF
OUT
) directly at the output pin.
F
and R
R
L
V
R
= 500Ω
OUT
G
L
V
= 500Ω
OUT
increase bandwidth and gain
T
) and an external capacitor
F
, R
G
) values
September 21, 2005
T
) to the
FN7458.1