Datasheet

ADN2871
Rev. A | Page 16 of 20
Resistor Setpoint
In resistor setpoint calibration, the current through the resistor
from PAVSET to ground is the I
MPD
current. The recommended
method for measuring the I
MPD
current is to place a small
resistor in series with the PAVSET resistor (or potentiometer)
and measure the voltage across this resistor, as shown in
Figure 34.
The I
MPD
current is then equal to this voltage divided by the
value of resistor used. In resistor setpoint calibration, PAVSET is
held to 1.2 V nominal; it is recommended that the sense resistor
be selected so that the voltage across the sense resistor does not
exceed 250 mV.
05228-029
V
CC
PHOTODIODE
ADN2871
PAVSET
R
μ C ADC
INPUT
Figure 34. Single Measurement of I
MPD
Across a
Sense Resistor in Resistor Setpoint I
MPD
Monitoring
LOOP BANDWIDTH SELECTION
To ensure that the ADN2871 control loop has sufficient
bandwidth, the average power loop capacitor (PAVCAP) is
calculated using the laser’s slope efficiency (watts/amps) and
the average power required.
For resistor setpoint control:
AV
P
LI
PAVCAP
××=
−6
102.3
(Farad)
For voltage setpoint control:
AV
P
LI
PAVCAP
××=
−6
1028.1
(Farad)
where:
P
AV
is the average power required (mW).
LI is the typical slope efficiency at 25°C of a batch of lasers that
are used in a design (mW/mA).
LI can be calculated as
MOD
I
P0P1
LI
−
=
(mW/mA)
where:
P1 is the optical power at the one level (mW).
P0 is the optical power at the zero level (mW).
The capacitor value equation is used to get a centered value for
the particular type of laser that is used in a design and an average
power setting. The laser LI can vary by a factor of 7 between
different physical lasers of the same type and across temperatures
without the need to recalculate the PAVCAP value.
This capacitor is placed between the PAVCAP pin and ground.
It is important that the capacitor is a low leakage, multilayer
ceramic type with an insulation resistance greater than 100 GΩ
or a time constant of 1000 seconds, whichever is less. Pick a
standard off-the-shelf capacitor value such that the actual
capacitance is within ±30% of the calculated value after the
capacitor’s own tolerance is taken into account.
POWER CONSUMPTION
The ADN2871 die temperature must be kept below 125°C. The
LFCSP has an exposed paddle, which should be connected so
that it is at the same potential as the ADN2871 ground pins.
Power consumption can be calculated as
I
CC
= I
CC
min + 0.3 I
MOD
P = V
CC
× I
CC
+ (I
BIAS
× V
BIAS_PIN
) + I
MOD
(V
MODP_PIN
+ V
MODN_PIN
)/2
T
DIE
= T
AMBIENT
+ θ
JA
× P
Thus, the maximum combination of
I
BIAS
+ I
MOD
must be
calculated, where:
I
CC
min = 30 mA, the typical value of I
CC
provided in Table 1
with
I
BIAS
= I
MOD
= 0.
T
DIE
is the die temperature.
T
AMBIENT
is the ambient temperature.
V
BIAS_PIN
is the voltage at the IBIAS pin.
V
MODP_PIN
is the voltage at the IMODP pin.
V
MODN_PIN
is the voltage at the IMODN pin.
AUTOMATIC LASER SHUTDOWN (Tx_DISABLE)
ALS (Tx_DISABLE) is an input that is used to shut down the
transmitter’s optical output. The ALS pin is pulled up internally
with a 6 kΩ resistor and conforms to SFP MSA specifications.
When ALS is logic high or when open, both the bias and
modulation currents are turned off. If an alarm has triggered,
and the bias and modulation currents are turned off, ALS can
be brought high and then low to clear the alarm.
BIAS AND MODULATION MONITOR CURRENTS
IBMON and IMMON are current-controlled current sources
that mirror a ratio of the bias and modulation current. The
monitor bias current (IBMON) and the monitor modulation
current (IMMON) should both be connected to ground
through a resistor to provide a voltage proportional to the bias
current and modulation current, respectively. When using a
microcontroller, the voltage developed across these resistors can
be connected to two of the ADC channels, making available a
digital representation of the bias and modulation current.