11
FN6160.2
March 29, 2006
AC Test Circuits
Figure
27C and
27D illustrate the optimum output load for
testing AC performance at 500
and 150 loads. Figure
27E illustrates the optimun output load for 50
and 75
cable-driving.
Application Information
General
Key features of the ISL59448 include a fixed gain of 2,
buffered high impedance analog inputs and excellent AC
performance at output loads down to 150
for video cable-
driving. The current feedback output amplifiers are stable
operating into capacitive loads.
For the best isolation and crosstalk rejection, all GND pins
and NIC pins must connect to the GND plane.
AC Design Considerations
High speed current-feed amplifiers are sensitive to
capacitance at the inverting input and output terminals. The
ISL59448 has an internally set gain of 2, so the inverting
input is not accessible. Capacitance at the output terminal
increases gain peaking (Figure
1) and pulse overshoot
(Fi
gures15 thru
18). The AC response of the ISL59448 is
optimized for a total capacitance of 1.1pF over the load
range of 150
to 500.
PC board trace length should be kept to a minimum in order
to minimize output capacitance and prevent the need for
controlled impedance lines. At 500MHz trace lengths
approaching 1” begin exhibiting transmission line behavior
and may cause excessive ringing if controlled impedance
traces are not used. Figure
27A shows the optimum inter-
stage circuit when the total output trace length is less than
the critical length of the highest signal frequency.
For applications where pulse response is critical and where
inter-stage distances exceed LCRIT, the circuit shown in
Figure 27B is recommended. Resistor RS constrains the capacitance seen by the amplifier output to the trace
capacitance from the output pin to the resistor. Therefore,
RS should be placed as close to the ISL59448 output pin as
possible. For inter-stage distances much greater than LCRIT,
the back-loaded circuit shown in Figure
27E should be used
with controlled impedance PCB lines, with RS and RL equal
to the controlled impedance.
For applications where inter-stage distances are long, but
pulse response is not critical, capacitor CS can be added to
low values of RS 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 RL
is still required but can be 500
or greater, resulting in a
much smaller attenuation factor.
Control Signals
S0, S1, ENABLE, LE, HIZ - These are binary coded,
TTL/CMOS compatible control inputs. The S0, S1 pins select
the inputs. All three amplifiers are switched simultaneously
from their respective inputs. The ENABLE, LE, HIZ pins are
used to disable the part to save power, latch in the last logic
state and three-state the output amplifiers, respectively. For
AC Test Circuits
FIGURE 27A. TEST CIRCUIT WITH OPTIMAL OUTPUT LOAD
FIGURE 27B. INTER-STAGE APPLICATION CIRCUIT
FIGURE 27C. 500 TEST CIRCUIT WITH 50
LOAD
FIGURE 27D. 150 TEST CIRCUIT WITH 50
LOAD
FIGURE 27E. BACKLOADED TEST CIRCUIT FOR 75
VIDEO
CABLE APPLICATION
ISL59448
*CL
50
VIN
500
, or
RL
1.1pF
or
75
150
*CL Includes PCB trace capacitance
VOUT
LCRIT
ISL59448
CS
50
VIN
500
, or
RL
or
75
75
LCRIT
RS
CL
ISL59448
RS
*CL
VIN
475
TEST
1.1pF
50
56.2
50
EQUIPMENT
*CL Includes PCB trace capacitance
LCRIT
ISL59448
RS
*CL
VIN
118
TEST
1.1pF
50
,or
86.6
50
EQUIPMENT
75
*CL Includes PCB trace capacitance
LCRIT
ISL59448
RS
*CL
VIN
50
or 75
TEST
1.1pF
50
or
75
50
or 75
EQUIPMENT
*CL Includes PCB trace capacitance
LCRIT
ISL59448