REV. 0

–10–

AD9660

Offset Current Generator

The offset current source allows the user to inject a fixed,

uncalibrated current into the laser diode. The offset current

source is set by an external resistor connected between OFF-

PULSE. See Figure 2 for a transfer function of the offset cur-

rent source.

The offset current may be used to increase the output current

provided by the bias and/or write loops after calibration. Alter-

natively, the offset current may be added during the calibration

of the bias loop and switched off after calibration to drop the

bias current below the knee of the laser diode power curve.

This is illustrated in Figure 10.

4

3

0

0

120

20

40

100

2

1

CONSTANT WRITE POWER

BIAS CALIBRATION POWER

0

25

CASE

50

BIAS

OFFSET

2

60

80

BIAS CURRENT IS BELOW THE KNEE OF THE LASER DIODE

MOD

BIAS

1

FORWARD CURRENT – mA

OPERATING

BIAS LEVEL

Figure 10. Laser Diode Current-to-Optical Power Curve

Illustrating Bias Below Diode Knee

AD9660 Layout Considerations

As in all high speed applications, proper layout is critical; it is

particularly important when both analog and digital signals are

involved. Analog signal paths should be kept as short as pos-

sible, and isolated from digital signals to avoid coupling in noise.

In particular, digital lines should be isolated from OUTPUT,

PIN SENSE, WRITE LEVEL, and BIAS LEVEL traces. Digi-

tal signal paths should also be kept short, and run lengths

matched to avoid propagation delay mismatch.

Layout of the ground and power supply circuits is also critical.

A single, low impedance ground plane will reduce noise on the

circuit ground. Power supplies should be capacitively coupled

to the ground plane to reduce noise in the circuit. 0.1

µF sur-

face mount capacitors, placed as close as possible to the

circuit boards allow designers to lay out signal traces without

interrupting the ground plane, and provide low impedance

power planes to further reduce noise.

Minimizing the Impedance of the Output Current Path

Because of the very high current slew that the AD9660 is

capable of producing (70+ mA in 1.5 ns), the inductance of the

output current path to and from the laser diode is critical. A

good layout of the output current path will yield high quality

light pulses with rise times of about 1.5 ns and less than 5%

overshoot. A poor layout can result in significant overshoot and

ringing. The most important guideline for the layout is to mini-

mize the impedance (mostly inductance) of the output current

path to the laser. It is important to recognize that the laser cur-

rent path is a closed loop. The figure illustrates the path that

current travels: (1) from the output pins of the AD9660 to the

anode of the laser, (2) through the laser to the cathode

(ground), (3) through the return path, (4) through the 0.1

µF

the current travels through the output driver circuitry of the

AD9660, and back to the output pins. The inductance of this

loop can be minimized by placing the laser as close to the

AD9660 as possible to keep the loop short, and by placing the

send and return paths on adjacent layers of the PC board to take

advantage of mutual coupling of the path inductances. This

mutual coupling effect is the most important factor in reducing

inductance in the current path.