Datasheet

ADN2872
Rev. 0 | Page 14 of 20
0
8013-040
V
CC
PHOTODIODE
ADN2872
PAVSET
R
MICROCONVERTER
ADC
INPUT
Figure 31. Single Measurement of I
MPD
Across a
Sense Resistor in Resistor Setpoint I
MPD
Monitoring
LOOP BANDWIDTH SELECTION
To ensure that the ADN2872 control loops have sufficient
bandwidth, the average power loop capacitor (PAVCAP) and
the extinction ratio loop capacitor (ERCAP) are calculated
using the laser slope efficiency and the average power required.
For resistor setpoint control,
AV
P
LI
PAVCAP 
6
102.3 (Farad)
2
PAVCAP
ERCAP 
(Farad)
For voltage setpoint control,
AV
P
LI
PAVCAP 
6
1028.1 (Farad)
2
PAVCAP
ERCAP 
(Farad)
where:
P
AV
(mW) is the average power required.
LI (mW/mA) is the typical slope efficiency at 25°C of a batch of
lasers that are used in a design.
The preceding capacitor estimation formulas are used to obtain
a centered value for the particular type of laser that is used in a
design and average power setting. Laser LI can vary by a factor
of 7 between different physical lasers of the same type and across
temperature without the need to recalculate the PAVCAP and
ERCAP values. In the ac coupling configuration, LI can be
calculated as
MOD
I
P0P1
LI


(mW/mA)
where
P1 is the optical power (mW) at the one level, and P0 is
the optical power (mW) at the zero level.
These capacitors are placed between the PAVCAP and ERCAP
pins and ground. It is important that these capacitors are low
leakage multilayer ceramics with an insulation resistance
greater than 100 GΩ or a time constant of 1000 sec, whichever
is less. The capacitor tolerance can be ±30% from the calculated
value to the available off-the-shelf value, including the capacitor’s
own tolerance.
POWER CONSUMPTION
The ADN2872 die temperature must be kept below 125°C. The
LFCSP package has an exposed paddle that should be connected
such that it is at the same potential as the ADN2872 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
where:
I
CC
min is 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.
Thus, the maximum combination of
I
BIAS
+ I
MOD
must be
calculated.
AUTOMATIC LASER SHUTDOWN (Tx_DISABLE)
ALS (Tx_DISABLE) is an input that is used to shut down the
transmitter 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 open, both the bias and modulation
currents are turned off.
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 micro-
controller, 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.
IBIAS PIN
ADN2872 has one on-chip, 800 Ωpull-up resistor. The current
sink from this resistor is V
IBIAS
dependent.
8.0
IBIASCC
UP
VV
I


(mA)
where
V
IBIAS
is the voltage measured at the IBIAS pin after setup
of one laser bias current, I
BIAS
. Usually, when set up, a maximum
laser bias current of 100 mA results in a
V
IBIAS
of about 1.2 V. In
a worst-case scenario,
V
CC
= 3.6 V, V
IBIAS
= 1.2 V, and I
UP
≤ 3 mA.
This on-chip resistor helps to damp out the low frequency
oscillation observed from some inexpensive lasers. If the on-
chip resistance does not provide enough damping, one external
R
Z
may be necessary (see Figure 32 and Figure 33).