Datasheet

ADN2872
Rev. 0 | Page 13 of 20
RESISTOR SETPOINT CALIBRATION
In resistor setpoint calibration, the PAVREF, ERREF, and RPAV
pins 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 ADN2872
ensures that both PAVSET and ERSET are kept 1.2 V above
GND. The PAVSET and ERSET resistors are given by:
SP
A
V
PAVSET
RP
R


V23.1
(Ω)
AV
C
W
CWMPD
ERSET
P
ER
ER
P
I
R





1
1
V23.1
_
(Ω)
where:
P
AV
(mW) is the average power required.
R
SP
(A/W) is the monitor photodiode responsivity.
P
CW
(mW) is the dc optical power specified on the laser data
sheet.
I
MPD_CW
(mA) is the MPD current at that specified P
CW
.
ER is the desired extinction ratio (ER = P1/P0).
I
MPD
MONITORING
I
MPD
monitoring can be implemented for voltage setpoint and
resistor setpoint as follows.
Voltage Setpoint
In voltage setpoint calibration, the following methods can be
used for I
MPD
monitoring.
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 28) 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 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.
08013-043
V
CC
PHOTODIODE
ADN2872
R
1kΩ
PAVSET
RPAV
MICROCONVERTER
ADC
INPUT
Figure 28. 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 29, 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 can be placed in series
with the photodiode cathode and V
CC
, as shown in Figure 30.
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 I
MPD
. 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.
08013-011
V
CC
LDPHOTODIODE
MICROCONVERTER
ADC
DIFFERENTIAL
INPUT
Rx
200Ω
10pF
PAVSET
ADN2872
Figure 29. Differential Measurement of I
MPD
Across a Sense Resistor
08013-012
V
CC
V
CC
LD
PHOTODIODE
MICROCONVERTER
ADC
INPUT
PAVSET
ADN2872
Rx
200Ω
Figure 30. Single Measurement of I
MPD
Across a Sense Resistor
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 31.
The I
MPD
current is then equal to this voltage divided by the
value of the 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.