Datasheet

ADN2873
Rev. 0 | Page 15 of 20
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
SPAV
PAVSET
RP
R
×
=
V2.1
(kΩ)
MOD
ERSET
I
R
100V2.1 ×
=
(kΩ)
where:
P
AV
is the average power required (mW).R
SP
is the optical
responsivity (in mA/mW).
I
MOD
is the modulation current required (mA).
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
MPD
MONITORING
I
MPD
monitoring can be implemented for voltage setpoint and
resistor setpoint as described in the following sections.
Voltage Setpoint
In voltage setpoint calibration, two methods can be used for
I
MPD
monitoring: measuring voltage at RPAV and measuring
I
MPD
across a sense resistor.
Method 1: Measuring Voltage at RPAV
The I
MPD
current is equal to the voltage at RPAV divided by the
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
MPD
reading when the laser is in shutdown or fail
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.
V
CC
PHOTODIODE
ADN2873
R
1kΩ
MICROCONVERTER
ADC
INPUT
PAVSET
RPAV
07493-031
Figure 31. Single Measurement of I
MPD
at RPAV in Voltage Setpoint Mode
Method 2: Measuring I
MPD
Across a Sense Resistor
The second method has the advantage of providing a valid I
MPD
reading at all times but has the disadvantage of requiring a
differential measurement across a sense resistor directly in
series with the I
MPD
. As shown in Figure 32, a small resistor, Rx,
is placed in series with the I
MPD
. 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, Rx, can be placed
in series with the photodiode cathode and V
CC
, as shown in
Figure 33. When choosing the value of the resistor, the user must
take into account the expected I
MPD
value in normal operation.
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.
V
CC
LDPHOTODIODE
MICROCONVERTER
A
DC DIFFERENTIAL
INPUT
200Ω
Rx
10pF
PAVSET
ADN2873
0
7493-032
Figure 32. Differential Measurement of I
MPD
Across a Sense Resistor
V
CC
V
CC
LD
PHOTODIODE
MICROCONVERTER
ADC
INPUT
200Ω
Rx
PAVSET
ADN2873
0
7493-033
Figure 33. Single Measurement of I
MPD
Across a Sense Resistor
Resistor Setpoint
In resistor setpoint calibration, the current through the resistor
from PAVSET to GND is the I
MPD
current. The recommended
method for measuring the I
MPD
current is to place a small resis-
tor 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.