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AD96606 Datasheet(PDF) 7 Page - Analog Devices

No. de pieza AD96606
Descripción Electrónicos  200 MHz Laser Diode Driver with Light Power Control
Download  12 Pages
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Fabricante Electrónico  AD [Analog Devices]
Página de inicio  http://www.analog.com
Logo AD - Analog Devices

AD96606 Datasheet(HTML) 7 Page - Analog Devices

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AD9660
REV. 0
–7–
Choosing CBIAS HOLD and CWRITE HOLD
Choosing values for the hold capacitors, CWRITE and CHOLD, is a
tradeoff between output current droop when the control loops
are open, and the time it takes to calibrate and recalibrate the
laser power when the loops are closed.
The amount of output current droop is determined by the value
of the hold capacitor and the leakage current at that node.
When either of the two control loops are open (WRITE CAL or
BIAS CAL logic LOW), the pin connections for the hold
capacitors (WRITE HOLD and BIAS HOLD) are high imped-
ance inputs. Leakage currents will range from
±200 nA; this
low current minimizes the droop in the output power level. As-
suming the worst case current of
±200 nA, the output current
will change as follows:
±∆I
BIAS =
18
×10
−9
t
BIAS HOLD
C
BIAS HOLD
±∆I
MODULATED =
18
×10
−9
t
WRITE HOLD
C
WRITE HOLD
To choose a value, the user will need to determine the amount
of time the loop will be in hold mode, tWRITE HOLD or tBIAS HOLD,
the maximum change in laser output power the application can
tolerate, and the laser efficiency (defined as the change in laser
output power to the change in laser diode current). As an ex-
ample, if an application requires 5 mW of laser power
±5%, and
the laser diode efficiency is 0.25 mW/mA, then
∆I
MAX = 5 mW × (5% ) /
0. 25
mW
mA


= 1.0 mA
If the same application had a hold time requirement of 250
µs,
then the minimum value of the hold capacitor would be:
C
HOLD =
18
×10
−9 × 250 µs
1.0 mA
= 4.5 nF
When determining the calibration time, the T/H and the exter-
nal hold capacitor can be modeled using the simple RC circuit
illustrated in Figure 7.
CHOLD
R
AD9660
TZA
T/H
WRITE LEVEL
OR BIAS LEVEL
POWER MONITOR
WRITE HOLD
OR BIAS HOLD
EXTERNAL HOLD
CAPACITOR
Figure 7. Circuit Model for Determining Calibration Times
Minimum specifications for IMONITOR MAX should be used when
choosing RGAIN. Users are cautioned that laser diode/photo di-
ode combinations that produce monitor currents that are less
than IMONITOR MAX in the equation above will produce higher la-
ser output power than predicted, which may damage the laser
diode. Such a condition is possible if RGAIN is calculated using
typical instead of minimum monitor current specifications. In
that case the input range to the AD9660 BIAS LEVEL and
WRITE LEVEL inputs should be limited to avoid damaging
laser diodes.
Although not recommended, another approach would be to
use a potentiometer for RGAIN. This allows users to optimize
the value of RGAIN for each laser diode/photo diode combina-
tion’s monitor current. The drawback to this approach is that
potentiometer’s stray inductance and capacitance may cause the
transimpedance amplifier to overshoot and degrade its settling,
and the value of CGAIN may not be optimized for the entire
potentiometer’s range.
CGAIN optimizes the response of the transimpedance amplifier
and should be chosen as from the table below. Choosing CGAIN
larger than the recommended value will slow the response of the
amplifier. Lower values improve TZA bandwidth but may cause
the amplifier to oscillate.
Table I.
Recommended
RGAIN
CGAIN
≥2.5 kΩ
2 pF
1.5 k
3 pF
1 k
4 pF
500
8 pF
The circuit in Figure 6 allows an adjustable gain with low vari-
ance in bandwidth, but requires several external components.
VREF
RINTERNAL
50
GAIN
C1
RF
CF
POWER
MONITOR
VREF
R1
R2
R3
1.7k
2I PIN
1:2
IMONITOR
SENSE
IN
AD9660
EQUIVALENT
CIRCUIT
REQ
VREF
RINTERNAL
50
CEQ
REQ + 50Ω = (RF + 50Ω)
(1+
+
)
R2
R1
R1
RF
Figure 6. Adjustable Gain Configuration


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