AD9661A

REV. 0

–7–

TIME CONSTANTS –

100

30

0

05

1

24

20

10

3

40

50

60

70

80

90

Figure 4. Calibration Time

Initial calibration is required after power-up or any other time

the laser has been disabled. Disabling the AD9661A drives the

hold capacitor to

≈V

output current is more than 10% out of calibration, R will range

from 300

Ω to 550 Ω for the model above; the higher value should

be used for calculating the worst case calibration time. Following

τ = RC = 550 Ω × 4.5 nF would be 2.48 µs. For an initial

calibration error < 1%, the initial calibration time should be

> 5

τ = 12.36 µs.

Initial calibration time will actually be better than this calcula-

tion indicates, as a significant portion of the calibration time will

be within 10% of the final value, and the output resistance in

the AD9660’s T/H decreases as the hold voltage approaches its

final value.

Recalibration is functionally identical to initial calibration, but

the loop need only correct for droop. Because droop is assumed

to be a small percentage of the initial calibration (< 10%), the

resistance for the model above will be in the range of 75

Ω to

140

Ω. Again, the higher value should be used to estimate the

worst case time needed for recalibration.

Continuing with the example above, since the droop error dur-

ing hold time is < 5%, we meet the criteria for recalibration and

τ = RC = 140 Ω × 4.5 nF = 0.64 µs. To get a final error of 1%

after recalibration, the 5% droop must be corrected to within a

20% error (20%

× 5% = 1%). A 2 τ recalibration time of 1.2 µs

is sufficient.

Continuous Recalibration

In applications where the hold capacitor is small (< 500 pF) and

the WRITE PULSE signals always have a pulse width > 25 ns,

the user may continuously calibrate the feedback loop. In such

an application, the

CAL signal should be held logic LOW, and

the PULSE signal will control loop calibration via the internal

AND gate. In such application, it is important to optimize the

Driving the Analog Inputs

The POWER LEVEL input of the AD9661A drives the track

and hold amplifier and allows the user to adjust the amount of

output current as described above. The input voltage range is

on board level shift circuit). The circuit below will perform the

level shift and scale the output of a DAC whose output is from

ground to a positive voltage. This solution is especially attrac-

tive 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.

DAC

V

DAC

OP191

+5V

V

= V

POWER LEVEL

R2

R1

BIAS LEVEL

V

REF

AD9661A

R1

R1

R2

R2

Figure 5. Driving the Analog Inputs

Using the Level Shift Circuit

The AD9661A includes an on board level shift circuit to provide

the offset described above. The input, LEVEL SHIFT IN, has

an input range from 0.1 V to 1.6 V. The output, LEVEL

drive POWER MONITOR. The linearity of the level shift cir-

cuit is poor for inputs below 100 mV. Between 100 mV and

1.6 V it is about 7 bits accurate.

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

possible, and isolated from digital signals to avoid coupling in

noise. In particular, digital lines should be isolated from

OUTPUT, SENSE IN, POWER LEVEL, LEVEL SHIFT IN

POWER MONITOR, and HOLD traces. Digital 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

surface mount capacitors, placed as close as possible to the

diode meet this requirement. Multilayer circuit boards allow

designers to lay out signal traces without interrupting the ground

plane, and provide low impedance power planes to further

reduce noise.