ISL59452 Datasheet, PDF (13/15 Pages) Intersil Corporation – Triple 4:1 Single Supply Video Multiplexing Amplifier

ISL59452
V_IN
50Ω
or
75Ω
ISL59452
x2
*C_L
2.1pF
R_L
150Ω
V_OUT
*C_L Includes PCB trace capacitance
FIGURE 32A. TEST CIRCUIT WITH OPTIMAL OUTPUT LOAD
ISL59452 _LCRIT
V_IN
x2
50Ω
R_S
or
C_L
C_S
R_L
75Ω
FIGURE 32B. INTER-STAGE APPLICATION CIRCUIT
V_IN
50Ω,or
75Ω
ISL59452
L_{CRIT RS}
x2
*C_L ^118Ω
2.1pF
86.6Ω
TEST
EQUIPMENT
50Ω
*C_L Includes PCB trace capacitance
FIGURE 32C. 150Ω TEST CIRCUIT WITH 50Ω LOAD
ISL59452
L_{CRIT RS}
V_IN
x2
50Ω
50Ω or 75Ω
or
*C_L
75Ω
2.1pF
TEST
EQUIPMENT
50Ω/75Ω
*C_L Includes PCB trace capacitance
FIGURE 32D. BACKLOADED TEST CIRCUIT FOR 150Ω VIDEO
CABLE APPLICATION
FIGURE 32. AC TEST CIRCUITS
AC Test Circuits
Figure 32A and 32B illustrate the optimum output load for
testing AC performance at 150Ω loads. Figure 32C
illustrates how to use the optimal 150Ω load for a 50Ω cable.
Figure 32D illustrates the optimum output load for 50Ω and
75Ω cable-driving.
Application Information
General
The ISL59452 triple 4:1 video MUX features +5V single-supply
operation, high bandwidth and TTL/CMOS logic compatible
gain select (AV2) of x1 (0dB) or x2 (+6dB). The ISL59452 also
features buffered high impedance analog inputs and excellent
AC performance at output loads down to 150Ω for video cable-
driving. The current feedback output amplifiers are stable
operating into capacitive loads.
AC Design Considerations
High speed current-feed amplifiers are sensitive to
capacitance at the inverting input and output terminals.
Capacitance at the output terminal increases gain peaking
and overshoot. The AC response of the ISL59452 is
optimized for a total output capacitance of 2.1pF with a load
of 150Ω (Figure 32A). When PCB trace capacitance and
component capacitance exceed 2pF, overshoot becomes
strongly dependent on the input pulse amplitude and slew
rate. Increasing levels of output capacitance reduce stability,
resulting in increased overshoot and settling time.
PC board trace length (L_CRIT) should be kept to a minimum
in order to minimize output capacitance. At 500MHz, trace
lengths approaching 1” begin exhibiting transmission line
behavior and may cause excessive ringing if controlled
impedance traces are not used. Figure 32B shows the
optimum inter-stage circuit when the total output trace length
is less than the critical length of the highest signal frequency.
As a general rule of thumb the trace lengths should be less
than one-tenth of the wavelength of the highest frequency
component in the signal. Equation 1 shows an approximate
way to calculate L_CRIT in meters.
L_C
R
IT
≤
---------------------^c----------------------
10 × f_MAX × ε_R
(EQ. 1)
c = speed of light (3 x 10^8 m/s)
f_MAX = maximum frequency component
ε_R = relative dielectric of board material (e.g. FR4 = 4.2)
For applications where inter-stage distances are long but
pulse response is not critical, capacitor C_S can be added to
low values of R_S to form a low-pass filter to dampen pulse
overshoot. This approach avoids the need for the large gain
correction required by the -6dB attenuation of the
back-loaded controlled impedance interconnect. Load
resistor R_L is still required but can be 500Ω or greater,
resulting in a much smaller attenuation factor.
For applications where pulse response is critical and where
inter-stage distances exceed L_CRIT, the circuit shown in
Figure 32C is recommended. Resistor R_S constrains the
capacitance seen by the amplifier output to the trace
capacitance betweeen the output pin and the resistor.
Therefore, R_S should be placed as close to the ISL59452
output pin as possible. For inter-stage distances much greater
than L_CRIT, the back-loaded circuit shown in Figure 32D
should be used with controlled impedance PCB lines, with R_S
and R_L equal to the controlled impedance.
Control Signals
S0, S1, AV2, and HIZ are binary coded, TTL/CMOS
compatible control inputs. The S0, S1 pins select the inputs.
All three output amplifiers are switched simultaneously from
their respective inputs. When HIZ is pulled high, it puts the
outputs in a high-impedance state. For control signal rise and
13
FN6254.0
September 24, 2007