Datasheet

ADN2871
Rev. A | Page 15 of 20
RESISTOR SETPOINT CALIBRATION
In resistor setpoint calibration, Pin PAVREF, Pin ERREF, and
Pin RPAV must all be tied to V
CC
. 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
ADN2871 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
(Ω)
MOD
ERSET
I
R
100V2.1 ×
=
(Ω)
where:
R
SP
is the optical responsivity (in amperes per watt).
I
MOD
is the modulation current required (mA).
P
AV
is the average power required (mW).
Power-On Sequence in Resistor Setpoint Mode
After power-on, the ADN2871 starts an initial process sequence
that takes 25 ms before enabling the alarms. Therefore, the
resistors connected to Pin PAVSET and Pin ERSET should be
stabilized within 20 ms after power-on. If the PAVSET and
ERSET resistors are connected to the ADN2871 20 ms after the
power supply is turned on, the ADN2871 alarm may kick in
and assert FAIL.
I
MPD
MONITORING
I
MPD
monitoring can be implemented for voltage setpoint and
resistor setpoint as described next.
Voltage Setpoint
In voltage setpoint calibration, two 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 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 capacitors should be placed on the RPAV node because
this can disturb the control loop.
05228-026
V
CC
PHOTODIODE
ADN2871
R
1kΩ
μ C ADC
INPUT
PAVSET
RPAV
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
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.
05228-027
V
CC
LDPHOTODIODE
µC ADC
DIFFERENTIAL
INPUT
200Ω
Rx
10pF
PAVSET
ADN2871
Figure 32. Differential Measurement of I
MPD
Across a Sense Resistor
05228-028
V
CC
V
CC
LD
PHOTODIODE
µC AD
C
INPUT
200Ω
Rx
PAVSET
ADN2871
Figure 33. Single Measurement of I
MPD
Across a Sense Resistor