ADN2872(RevB) Просмотр технического описания (PDF) - Analog Devices
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ADN2872 Datasheet PDF : 20 Pages
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Data Sheet
Operation with Lasers with Temperature-Dependent
Nonlinearity of Laser LI Curve
The ADN2872 ERCL extracts information from the monitor
photodiode signal relating to the slope of the LI characteristics
at the Optical 1 level (P1). For lasers with good linearity over
temperature, the slope measured by the ADN2872 at the Optical 1
level is representative of the slope anywhere on the LI curve.
This slope information sets the required modulation current to
achieve the required optical ER.
4.0
RELATIVELY LINEAR LI CURVE AT 25°C
3.5
3.0
2.5
2.0
1.5
1.0
NONLINEAR LI CURVE AT 80°C
0.5
0
0
20
40
60
80
100
CURRENT (mA)
Figure 25. Measurement of a Laser LI Curve Showing
Laser Nonlinearity at High Temperatures
Some types of lasers have LI curves that become progressively
more nonlinear with increasing temperature (see Figure 25). At
temperatures where the LI curve shows significant nonlinearity,
the LI curve slope measured by the ADN2872 at the Optical 1
level is no longer representative of the overall LI curve. It is evident
that applying a modulation current based on this slope information
cannot maintain a constant ER over temperature.
However, the ADN2872 can be configured to maintain near
constant optical bias and an ER with a laser exhibiting a monotonic
temperature-dependent nonlinearity. To implement this correction,
it is necessary to characterize a small sample of lasers for their
typical nonlinearity by measuring them at two temperature points,
typically 25°C and 85°C. The measured nonlinearity determines
the amount of feedback to apply.
Typically, the user must characterize five to 10 lasers of a particular
model to obtain a good number. The product can then be cali-
brated at 25°C only, avoiding the expense of temperature
calibration. Typically, the microcontroller measures the laser and
apply the feedback. This scheme is particularly suitable for circuits
that already use a microcontroller for control and digital diagnostic
monitoring.
ADN2872
The ER correction scheme, while using the average nonlinearity
for the laser population, supplies a corrective measurement based
on the actual performance of each laser as measured during
operation. The ER correction scheme corrects for errors due to
laser nonlinearity while the dual loop continues to adjust for
changes in the Laser LI.
For more details on maintaining average optical power and
ER over temperature when working with lasers displaying a
temperature-dependent nonlinearity of LI curve, contact sales
at Analog Devices.
CONTROL
The ADN2872 has two methods for setting the average power
(PAV) and ER. The average power and ER can be voltage set
using the voltage DAC outputs of a microcontroller to provide
controlled reference voltages to PAVREF and ERREF. Alternatively,
the average power and ER can be resistor set using potentiometers
at the PAVSET and ERSET pins, respectively.
VOLTAGE SETPOINT CALIBRATION
The ADN2872 allows an interface to a microcontroller for both
control and monitoring (see Figure 26). The average power at
the PAVSET pin and ER at the ERSET pin can be set using the
DAC of the microcontroller to provide controlled reference
voltages to PAVREF and ERREF. Note that during power-up,
there is an internal sequence that allows 25 ms before enabling
the alarms; therefore, the user must ensure that the voltage for
PAVREF and ERREF are active within 20 ms.
PAVREF = PAV × RSP × RPAV
(V)
ERREF
= RERSET
× I MPD _CW
PCW
×
ER
ER
−
+
1
1
×
PAV
(V)
where:
PAV (mW) is the average power required.
ER is the desired extinction ratio (ER = P1/P0).
RSP (A/W) is the monitor photodiode responsivity.
IMPD_CW (mA) is the MPD current at that specified PCW.
PCW (mW) is the dc optical power specified on the laser data
sheet.
In voltage setpoint, RPAV and RERSET must be 1 kΩ resistors with
a 1% tolerance and a temperature coefficient of 50 ppm/°C.
Rev. B | Page 11 of 20
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