AD9660

REV. 0

–9–

Example Calculations

The example below (in addition to the one included in the sec-

hold capacitor values, and worst case calibration times.

System Requirements:

• Bias laser power: 4 mW Bias ± 5%

• Write laser power: 25 mW ± 0.5%

• Bias Hold Time: 1 ms

• Write Hold Time: 1 µs

Laser diode/photo diode characteristics:

• Laser efficiency 0.5 mA/mA

• Monitor current: 5 µA/mA

• From the laser power requirements and efficiency we can

estimate:

∆I

0. 5

mW

mA









= 400.0 µA

and

∆I

(

mA









= 250 µA

• Choosing hold caps based on these:

C

18

×10

400

µA

= 0.045 µF

and

C

18

×10

250

µA

= 72 pF

• The bias loop initial calibration time for a <1% error:

5

τ = 5 × RC = 5 × 550

Ω × 0.045 µF = 123.75 µs

• Bias loop recalibration for a 1% error after 5% droop (need to

correct within 20%):

2

τ = 2 × RC = 2 × 140

Ω × 0.045 µF = 12.6 µs

• The write loop initial calibration time for <0.1% error:

7

τ = 7 × RC = 7 × 550

Ω × 72 pF = 277.2 ns

• Write loop re-calibration for a 0.1% error after 0.5% droop

(need to correct within 20%):

2

τ = RC = 2 × 140

Ω × 72 pF = 20.2 ns

• From the monitor current specification and the max power

specified:

5

µA

mW

= 125 µA

and

R

1.6 V

1.85

× I

MONITOR MAX

− 50 Ω= 6.9 kΩ

• C

Driving the Analog Inputs

The BIAS LEVEL and WRITE LEVEL inputs of the AD9660

drive the track and hold amplifiers and allow the user to adjust

the amount of output current as described above. The input

The circuit in Figure 9 will perform the level shift and scale the

output of a DAC whose output is from ground to a positive

voltage. This solution is attractive because both the DAC and

the op amp can run off a single +5 V supply, and the op amp

doesn’t have to swing rail to rail.

DAC1

DAC2

OP291

OP291

+5V

R2

R1

R4

R3

BIAS LEVEL

WRITE LEVEL

AD9660

R1

R2

R3

R4

R3

R4

R1

R2

Figure 9. Driving the Analog Inputs