ADN2870ACPZ Analog Devices Inc, ADN2870ACPZ Datasheet - Page 14

• Manufacturer Part Number: ADN2870ACPZ
• Description: IC,Laser Diode/LED Driver,LLCC,24PIN,PLASTIC
• Manufacturer: Analog Devices Inc
• Type: Laser Diode Driver (Fiber Optic)r

Specifications of ADN2870ACPZ

• Data Rate: 3.3Gbps
• Number Of Channels: 1
• Voltage - Supply: 3 V ~ 3.6 V
• Current - Supply: 30mA
• Current - Modulation: 90mA
• Current - Bias: 100mA
• Operating Temperature: -40°C ~ 85°C
• Package / Case: 24-VFQFN, CSP Exposed Pad
• Mounting Type: Surface Mount
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

ADN2870
RESISTOR SETPOINT CALIBRATION
In resistor setpoint calibration, 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 ADN2870
ensures that both PAVSET and ERSET are kept 1.2 V above
GND. The resistors connected to PAVSET and ERSET are
given by
where:
R
P
sheet.
I
P
ER is the desired extinction ratio (ER = P1/P0).
I
I
resistor setpoint as described in the sections that follow.
Voltage Setpoint
In voltage setpoint calibration, the following methods can be
used for I
Method 1: Measuring Voltage at RPAV
The I
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
fail mode. A microconverter-buffered A/D 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.
MPD_CW
MPD
MPD
CW
AV
SP
Figure 28. Single Measurement of I
(A/W) is the monitor photodiode responsivity.
(mW) is the average power required.
(mW) is the dc optical power specified on the laser data
monitoring can be implemented for voltage setpoint and
MONITORING
R
R
MPD
PAVSET
ERSET
(mA) is MPD current at that specified P
current is equal to the voltage at RPAV divided by the
MPD
=
monitoring.
=
I
MPD
P
P
MPD
AV
. 1
CW
PHOTODIODE
_
23
µC ADC
CW
×
reading when the laser is in shutdown or
INPUT
R
V
. 1
×
SP
23
ER
ER
V
−
+
V
1
1
CC
MPD
R
1kΩ
×
P
at RPAV in Voltage Setpoint Mode
AV
PAVSET
RPAV
ADN2870
CW
.
Rev. A | Page 14 of 20
(Ω)
(Ω)
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 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 I
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 I
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 I
method for measuring the I
resistor in series with the PAVSET resistor (or potentiometer)
and measure the voltage across this resistor, as shown in Figure 31.
The I
value of resistor used. In resistor setpoint, 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.
Figure 29. Differential Measurement of I
MPD
Figure 30. Single Measurement of I
current is then equal to this voltage divided by the
MPD
DIFFERENTIAL
µC ADC
. As shown in Figure 29, a small resistor, Rx,
µC ADC
PHOTODIODE
INPUT
INPUT
PHOTODIODE
MPD
RESISTOR
ADN2870
MPD
MPD
Across a Sense Resistor
200Ω
ADN2870
. If the laser used in the design
MPD
current is to place a small
200Ω
RESISTOR
PAVSET
V
CC
current. The recommended
V
MPD
PAVSET
CC
MPD
MPD
Across a Sense Resistor
V
Across a Sense Resistor
CC
value in normal
LD
LD
10pF
MPD
MPD
.