11

FN6160.2

March 29, 2006

AC Test Circuits

Figure 27C and 27D illustrate the optimum output load for

testing AC performance at 500

Ω and 150Ω loads. Figure

27E illustrates the optimun output load for 50

Ω and 75Ω

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

Ω 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

(Figures15 thru 18). The AC response of the ISL59448 is

optimized for a total capacitance of 1.1pF over the load

range of 150

Ω to 500Ω.

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

capacitance seen by the amplifier output to the trace

capacitance from the output pin to the resistor. Therefore,

the back-loaded circuit shown in Figure 27E should be used

to the controlled impedance.

For applications where inter-stage distances are long, but

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

Ω 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

Ω LOAD

Ω LOAD

FIGURE 27E. BACKLOADED TEST CIRCUIT FOR 75

Ω VIDEO

CABLE APPLICATION

ISL59448

50

Ω

500

Ω, or

1.1pF

or

75

Ω

150

Ω

ISL59448

50

Ω

500

Ω, or

or

75

Ω

75

Ω

ISL59448

475

Ω

TEST

1.1pF

50

Ω

56.2

Ω

50

Ω

EQUIPMENT

ISL59448

118

Ω

TEST

1.1pF

50

Ω,or

86.6

Ω

50

Ω

EQUIPMENT

75

Ω

ISL59448

50

Ω or 75Ω

TEST

1.1pF

50

Ω

or

75

Ω

50

Ω or 75Ω

EQUIPMENT

ISL59448