ISL1591 Datasheet, PDF (14/18 Pages) Intersil Corporation – Fixed Gain, Dual Port, VDSL2 Line Driver

Test Circuit
A
R
NETWORK
ANALYZER
S
DC
SPLITTER
ISL1591
+12
50Ω
180°
SPLITTER
DUT
243Ω
1:1
R_L
56Ω
243Ω
50Ω
LOAD
50Ω
-12
FIGURE 48. FREQUENCY RESPONSE CHARACTERIZATION CIRCUIT
Applications Information
Applying Wideband Current Feedback Op
Amps as Differential Drivers
A current feedback amplifier (CFA) like the ISL1591 is
particularly suited to the requirements of high output power, full
power bandwidth, differential drivers. This topology offers a very
high slew rate on low quiescent power. The ISL1591 is a fixed
gain amplifier set to an optimized gain of 11.6V/V given
R_L = 82.6 and R_b = 8.25, as shown in Figure 49.
The ISL1591 provides 4 very power efficient, high output current,
CFA's. These are intended to be connected as two pairs of
differential drivers. The “Connection Diagram” on page 2 shows
that Channels A and B are intended to operate as a pair while
Channels C and D comprise the other pair. Power control is also
provided through two pairs of control pins, which separately set
the power for Channels A and B together and then the other pair
controls Channels C and D together.
Very low output distortion can be provided by the differential
configuration. The high slew rate intrinsic to the CFA topology
also contributes to the exceptional performance shown in
Figures 9, 15 and 21. These swept frequency distortion plots
show extremely low distortion at 200kHz holding to very low
levels up through 20MHz. At the full operating power, (Figure 9,
7mA per amplifier or 14mA/port) we still see < -70dBc through
5MHz for a 5V_P-P differential output swing.
Advanced Configurations - Active
Termination
Where the best power efficiency is required in a full duplex DSL
line interface application, it is common to apply the circuit shown
in Figure 49 to reduce the power loss in the matching loads, R_b,
while retaining a higher impedance for the upstream signal
coming into this output stage. This circuit acts to provide a higher
apparent output impedance (through its cross-coupled positive
feedback through the R_p resistors integrated internally) while
physically taking a smaller I_R drop through the R_b resistors for
the output signal.
14
FN7625.1
October 31, 2012