ADN2870ACPZ-RL7 Analog Devices Inc, ADN2870ACPZ-RL7 Datasheet - Page 12
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ADN2870ACPZ-RL7
Manufacturer Part Number
ADN2870ACPZ-RL7
Description
IC,Laser Diode/LED Driver,LLCC,24PIN,PLASTIC
Manufacturer
Analog Devices Inc
Type
Laser Diode Driver (Fiber Optic)r
Datasheet
1.ADN2870ACPZ.pdf
(20 pages)
Specifications of ADN2870ACPZ-RL7
Data Rate
3.3Gbps
Number Of Channels
1
Voltage - Supply
3 V ~ 3.6 V
Current - Supply
30mA
Current - Modulation
90mA
Current - Bias
100mA
Operating Temperature
-40°C ~ 85°C
Package / Case
24-VFQFN, CSP Exposed Pad
Mounting Type
Surface Mount
Lead Free Status / RoHS Status
Lead free / RoHS Compliant
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Price
ADN2870
Operation with Lasers with Temperature-Dependent
Nonlinearity of Laser LI Curve
The ADN2870 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 ADN2870 at the Optical 1
level is representative of the slope anywhere on the LI curve.
This slope information is used to set the required modulation
current to achieve the required optical extinction ratio.
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 ADN2870 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 extinction ratio over
temperature.
However, the ADN2870 can be configured to maintain near
constant optical bias and an extinction ratio 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 is used to determine the
amount of feedback to apply.
Typically, one must characterize 5 to 10 lasers of a particular
model to get a good number. The product can then be cali-
brated at 25°C only, avoiding the expense of temperature
calibration. Typically, the microcontroller supervisor is used
to measure the laser and apply the feedback. This scheme is
particularly suitable for circuits that already use a microcon-
troller for control and digital diagnostic monitoring.
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
0
Figure 25. Measurement of a Laser LI Curve Showing
RELATIVELY LINEAR LI CURVE AT 25°C
Laser Nonlinearity at High Temperatures
20
CURRENT (mA)
40
NONLINEAR LI CURVE AT 80°C
60
80
100
Rev. A | Page 12 of 20
The ER correction scheme, while using the average nonlinearity
for the laser population, in fact, supplies a corrective measure-
ment based on each laser’s actual performance 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
extinction ratio over temperature when working with lasers
displaying a temperature-dependent nonlinearity of LI curve,
refer to the Application Note AN-743 available through Analog
Devices sales.
CONTROL
The ADN2870 has two methods for setting the average power
(P
extinction ratio can be voltage set using a microcontroller’s
voltage DAC outputs to provide controlled reference voltages to
PAVREF and ERREF. Alternatively, the average power and
extinction ratio can be resistor set using potentiometers at the
PAVSET and ERSET pins, respectively.
VOLTAGE SETPOINT CALIBRATION
The ADN2870 allows an interface to a microcontroller for both
control and monitoring (see Figure 26). The average power at
the PAVSET pin and extinction ratio at the ERSET pin can be
set using the microcontroller’s DAC to provide controlled
reference voltages PAVREF and ERREF. After power-on, the
ADN2870 starts an initial process that takes 25 ms before
enabling the alarms; therefore, the customer must ensure that
stable reference voltages to PAVREF and ERREF are available
within 20 ms.
where:
R
P
sheet.
I
P
ER is the desired extinction ratio (ER = P1/P0).
In voltage setpoint, RPAV and R
a 1% tolerance and a temperature coefficient of 50 ppm/°C.
MPD_CW
CW
AV
SP
AV
(A/W) is the monitor photodiode responsivity.
(mW) is the average power required.
) and extinction ratio (ER). The average power and
(mW) is the dc optical power specified on the laser data
PAVREF = P
ERREF
(mA) is the MPD current at the specified P
=
R
ERSET
AV
× R
×
SP
I
MPD
× RPAV
P
CW
_
CW
ERSET
×
ER
ER
must be 1 kΩ resistors with
−
+
1
1
×
P
AV
CW
.
(Volts)
(Volts)