ISL59421 Ver la hoja de datos (PDF) - Renesas Electronics
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ISL59421 Datasheet PDF : 12 Pages
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ISL59421
Application Circuits
357
VIN
50
-
+
0.6pF
CG
PC BOARD
CAPACITANCE
*CL = CT + COUT
CT
1.6pF
COUT
3.3pF
VOUT
RL = 500
0.4pF < CG < 0.7pF
*CL: TOTAL LOAD CAPACITANCE
CT: TRACE CAPACITANCE
COUT: OUTPUT CAPACITANCE
FIGURE 25A. GAIN OF 1 APPLICATION CIRCUIT
100
VIN
50
100
-
+
0.6pF
CG
PC BOARD
CAPACITANCE
*CL = CT + COUT
CT
1.6pF
COUT
4.7pF
0.4pF < CG < 0.7pF
FIGURE 25B. GAIN OF 2 APPLICATION CIRCUIT
VOUT
RL = 500
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 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 or 75 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 (CF), and to ground (CG),
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 (CG). Capacitance
(CF) to the output should be minimized. To 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
(CL) directly on the output pin. Increased capacitance 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 (CT) and an external capacitor (COUT). A
good method to maintain control over the output pin
capacitance is to minimize the trace length (CT) to the next
component, and include a discrete surface mount capacitor
(COUT) directly at the output pin.
Feedback Resistor Values
The AC performance of the output amplifier is optimized with
the feedback resistor network (RF, RG) values 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 RF and RG
increase bandwidth and gain 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.
FN7458 Rev 1.00
September 21, 2005
Page 9 of 12
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