ISL1591

14

FN7625.1

October 31, 2012

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

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

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

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

the output signal.

Test Circuit

FIGURE 48. FREQUENCY RESPONSE CHARACTERIZATION CIRCUIT

180°

SPLITTER

DC

SPLITTER

50

Ω

50

Ω

-12

+12

DUT

243

Ω

243

Ω

56

Ω

1:1

50

Ω

A

R

S

NETWORK

ANALYZER

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