ADN2870 Analog Devices, Inc., ADN2870 Datasheet - Page 14

• Manufacturer Part Number: ADN2870
• Description: 3.3 V Dual-loop, 50 Mbps To 3.3 Gbps Laser Diode Driver
• Manufacturer: Analog Devices, Inc.

ADN2870
RESISTOR SETPOINT CALIBRATION
In resistor setpoint calibration. PAVREF, ERREF, and RPAV
must all be tied to VCC. Average power and extinction ratio can
be set using the PAVSET and ERSET pins, respectively. A
resistor is placed between the pin and GND to set the current
flowing in each pin as shown in Figure 27. The ADN2870
ensures that both PAVSET and ERSET are kept 1.2 V above
GND. The PAVSET and ERSET resistors are given by the
following:
where:
R
P
I
P
ER is the desired extinction ratio (ER = P1/P0).
IMPD MONITORING
IMPD monitoring can be implemented for voltage setpoint and
resistor setpoint as follows.
Voltage Setpoint
In voltage setpoint calibration, the following methods may be
used for IMPD monitoring.
Method 1: Measuring Voltage at RPAV
The IMPD current is equal to the voltage at RPAV divided by
the value of RPAV (see Figure 28) as long as the laser is on and
is being controlled by the control loop. This method does not
provide a valid IMPD reading when the laser is in shut-down or
fail mode. A microconverter buffered A/D input may be con-
nected to RPAV to make this measurement. No decoupling or
filter capacitors should be placed on the RPAV node because
this can disturb the control loop.
MPD_CW
CW
AV
SP
Figure 28. Single Measurement of IMPD RPAV in Voltage Setpoint Mode
is the monitor photodiode responsivity.
is the average power required.
is the dc optical power specified on the laser data sheet.
R
R
PAVSET
ERSET
is MPD current at that specified P
=
=
I
P
MPD
AV
P
. 1
PHOTODIODE
CW
23
µ
_
×
C ADC
INPUT
CW
R
V
SP
. 1
×
23
(Ω)
ER
ER
V
VCC
−
+
1
1
R
1kΩ
+
P
PAVSET
RPAV
ADN2870
AV
(Ω)
CW
.
Rev. 0 | Page 14 of 20
Method 2: Measuring IMPD Across a Sense Resistor
The second method has the advantage of providing a valid IMPD
reading at all times, but has the disadvantage of requiring a
differential measurement across a sense resistor directly in
series with the IMPD. As shown in Figure 29, a small resistor,
Rx, is placed in series with the IMPD. If the laser used in the
design has a pinout where the monitor photodiode cathode and
the lasers anode are not connected, a sense resistor can be placed
in series with the photodiode cathode and VCC as shown in
Figure 30. When choosing the value of the resistor, the user
must take into account the expected IMPD value in normal
operation. The resistor must be large enough to make a signifi-
cant signal for the buffered A/Ds to read, but small enough so as
not to cause a significant voltage reduction across the IMPD.
The voltage across the sense resistor should not exceed 250 mV
when the laser is in normal operation. It is recommended that a
10 pF capacitor be placed in parallel with the sense resistor.
Resistor Setpoint
In resistor setpoint calibration, the current through the resistor
from PAVSET to ground is the IMPD current. The recommended
method for measuring the IMPD current is to place a small
resistor in series with PAVSET resistor (or potentiometer) and
measure the voltage across this resistor as shown in Figure 31.
The IMPD current is then equal to this voltage divided by the
value of resistor used. In resistor setpoint, PAVSET is held to
1.2 V nominal; it is recommended that the sense resistor should
be selected so that the voltage across the sense resistor does not
exceed 250 mV.
Figure 29. Differential Measurement of IMPD Across a Sense Resistor
Figure 30. Single Measurement of IMPD Across a Sense Resistor
DIFFERENTIAL
µ
µ
C ADC
PHOTODIODE
INPUT
INPUT
C ADC
PHOTODIODE
RESISTOR
ADN2870
200Ω
ADN2870
200Ω
RESISTOR
PAVSET
VCC
VCC
PAVSET
VCC
LD
LD
10pF