ADN2873ACPZ-RL7 AD [Analog Devices], ADN2873ACPZ-RL7 Datasheet - Page 15

• Manufacturer Part Number: ADN2873ACPZ-RL7
• Description: 3.3 V, 50 Mbps to 4.25 Gbps, Single-Loop, Laser Diode Driver
• Manufacturer: AD [Analog Devices]

RESISTOR SETPOINT CALIBRATION
In resistor setpoint calibration, the PAVREF, ERREF, and RPAV
pins must all be tied to VCC. The 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 30. The ADN2873
ensures that both PAVSET and ERSET are kept 1.23 V above
GND. The PAVSET and ERSET resistors are given by
where:
P
responsivity (in mA/mW).
I
Power-On Sequence in Resistor Setpoint Mode
Note that during power-up, the ADN2873 starts an initial process
sequence that allows 25 ms before enabling the device alarms.
The resistors connected to the PAVSET and ERSET pins should
be stable within 20 ms after turning on the power supply. The
ADN2873 alarm may kick in and assert FAIL, provided the
PAVSET and ERSET resistors are stabilized 20 ms after turning
on the power supply.
I
I
resistor setpoint as described in the following sections.
Voltage Setpoint
In voltage setpoint calibration, two methods can be used for
I
I
Method 1: Measuring Voltage at RPAV
The I
value of RPAV (see Figure 31) as long as the laser is on and is
being controlled by the control loop. This method does not
provide a valid I
mode. A microconverter-buffered ADC input can be connected to
RPAV to make this measurement. No decoupling or filter capaci-
tors should be placed on the RPAV node because this can disturb
the control loop.
MOD
MPD
MPD
MPD
MPD
AV
Figure 31. Single Measurement of I
is the average power required (mW).R
monitoring can be implemented for voltage setpoint and
monitoring: measuring voltage at RPAV and measuring
across a sense resistor.
is the modulation current required (mA).
MONITORING
R
R
MPD
PAVSET
ERSET
current is equal to the voltage at RPAV divided by the
=
MICROCONVERTER
=
1
2 .
P
MPD
AV
1
I
V
MOD
2 .
×
×
reading when the laser is in shutdown or fail
PHOTODIODE
V
R
100
SP
INPUT
ADC
MPD
V
CC
R
1kΩ
at RPAV in Voltage Setpoint Mode
PAVSET
RPAV
ADN2873
SP
is the optical
(kΩ)
(kΩ)
Rev. 0 | Page 15 of 20
Method 2: Measuring I
The second method has the advantage of providing a valid I
reading at all times but has the disadvantage of requiring a
differential measurement across a sense resistor directly in
series with the I
is placed in series with the I
has a pinout where the monitor photodiode cathode and the
lasers anode are not connected, a sense resistor, Rx, can be placed
in series with the photodiode cathode and V
Figure 33. When choosing the value of the resistor, the user must
take into account the expected I
The resistor must be large enough to make a significant signal
for the buffered ADC to read, but small enough not to cause a
significant voltage reduction across the photodiode. 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 GND is the I
method for measuring the I
tor in series with the PAVSET resistor (or potentiometer) and
measure the voltage across this resistor, as shown in Figure 34. The
I
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.
MPD
Figure 32. Differential Measurement of I
current is then equal to this voltage divided by the value of
Figure 33. Single Measurement of I
MICROCONVERTER
ADC DIFFERENTIAL
MICROCONVERTER
MPD
. As shown in Figure 32, a small resistor, Rx,
INPUT
PHOTODIODE
INPUT
MPD
ADC
ADN2873
PHOTODIODE
MPD
MPD
Across a Sense Resistor
MPD
. If the laser used in the design
current is to place a small resis-
MPD
200Ω
Rx
PAVSET
current. The recommended
200Ω
ADN2873
MPD
value in normal operation.
V
Rx
CC
V
MPD
Across a Sense Resistor
CC
PAVSET
Across a Sense Resistor
LD
V
10pF
CC
CC
LD
, as shown in
ADN2873
MPD