ISL59440_07 Datasheet, PDF (9/12 Pages) Intersil Corporation – 400MHz Multiplexing Amplifier

ISL59440
Application Circuits
V_IN
50Ω
C_F
332Ω
-
+
0.6pF
C_g
PC BOARD
CAPACITANCE
0.4pF < C_G < 0.7pF
*C_L = C_T + C_OUT
C_T
1.6pF
C_OUT
0pF
V_OUT
R_L = 500Ω
*C_L: TOTAL LOAD CAPACITANCE
C_T: TRACE CAPACITANCE
C_OUT: OUTPUT CAPACITANCE
FIGURE 25A. GAIN OF 1 APPLICATION CIRCUIT
V_IN
50Ω
511Ω
511Ω
-
+
0.6pF
C_G
PC BOARD
CAPACITANCE
*C_L = C_T + C_OUT
C_T
1.6pF
C_OUT
3.9pF
0.4pF < C_G < 0.7pF
FIGURE 25B. GAIN OF 2 APPLICATION CIRCUIT
V_OUT
R_L = 500Ω
Application Information
General
The ISL59440 is a 4:1 mux that is ideal as a matrix element
in high performance switchers and routers. The ISL59440 is
optimized to drive 5pF in parallel with a 500Ω load. The
capacitance can be split between the PCB capacitance and
an external load capacitance. Its low input capacitance and
high input resistance provides 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 effected by stray
capacitance at the inverting input. Stray capacitance from
the inverting input pin to the output (C_F), and to ground (C_G),
increase gain peaking and bandwidth. Large values of either
capacitance can cause oscillation. The ISL59440 has been
optimized for a 0.4pF to 0.7pF capacitance (C_G).
Capacitance (C_F) 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
(C_L) 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 (C_T) and an external capacitor
(C_OUT). A good method to maintain control over the output
pin capacitance is to minimize the trace length (C_T) to the
next component, and include a discrete surface mount
capacitor (C_OUT) directly at the output pin.
FEEDBACK RESISTOR VALUES
The AC performance of the output amplifier is optimized with
the feedback resistor network (R_F, R_G) values
recommended in the application circuits. The amplifier
bandwidth and gain peaking are directly effected 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_F and R_G increase bandwidth and gain
peaking. This has the effect of decreasing rise/fall times and
increasing overshoot.
9
FN6162.2
October 10, 2007