ADN2848ACP-32 Analog Devices Inc, ADN2848ACP-32 Datasheet - Page 7

• Manufacturer Part Number: ADN2848ACP-32
• Description: IC DRVR LASER DIODE 3.3V 32LFCSP
• Manufacturer: Analog Devices Inc
• Type: Laser Diode Driver (Fiber Optic)r

Specifications of ADN2848ACP-32

• Number Of Channels: 1
• Rohs Status: RoHS non-compliant
• Data Rate: 1.25Gbps
• Voltage - Supply: 3 V ~ 3.6 V
• Current - Supply: 50mA
• Current - Modulation: 80mA
• Current - Bias: 100mA
• Operating Temperature: -40°C ~ 85°C
• Package / Case: 32-VFQFN, CSP Exposed Pad
• Mounting Type: Surface Mount
• Operating Supply Voltage (min): 3V
• Mounting: Surface Mount
• Lead Free Status / RoHS Status: Not Compliant

THEORY OF OPERATION
A laser diode (LD) has current-in to light-out transfer functions, as
shown in Figure 4. Two key characteristics of this transfer function
are the threshold current, I
the threshold current, referred to as slope efficiency, or LI.
CONTROL
A monitor photodiode, MPD, is required to control the LD. The
MPD current is fed into the ADN2848 to control the power and
extinction ratio, continuously adjusting the bias current and
modulation current in response to the laser’s changing
threshold current and light-to-current slope efficiency.
The ADN2848 uses automatic power control, APC, to maintain
a constant average power over time and temperature.
The ADN2848 uses closed-loop extinction ratio control to
allow optimum setting of extinction ratio for every device.
Thus, SONET/SDH interface standards can be met over device
variation, temperature, and laser aging. Closed-loop
modulation control eliminates the need to either overmodulate
the LD or include external components for temperature
compensation. This reduces research and development time
and second sourcing issues caused by characterizing LDs.
Average power and extinction ratio are set using the PSET and
ERSET pins, respectively. Potentiometers are connected
between these pins and ground. The potentiometer R
to change the average power. The potentiometer R
to adjust the extinction ratio. Both PSET and ERSET are kept
1.2 V above GND.
For an initial setup, R
calculated using the following formulas:
R
R
PSET
ERSET
=
=
P
1
P1
P0
AV
2 .
I
I
AV
MPD
V
P
Figure 4. Laser Transfer Function
CW
ER =
P
_
( )
AV
Ω
CW
PSET
=
P1
P0
P1 + P0
1
×
and R
TH
2 .
2
ER
ER
, and slope in the linear region beyond
V
−
+
ERSET
1
1
I
TH
×
potentiometers can be
P
ΔI
AV
ΔP
CURRENT
( )
Ω
LI =
ΔP
ΔI
ERSET
PSET
is used
is used
Rev. A | Page 7 of 12
where:
I
P
I
P
ER is the desired extinction ratio (ER = P1/P0).
Note that I
the control loops determine the actual values. It is not required
to know the exact values for LI or MPD optical coupling.
LOOP BANDWIDTH SELECTION
For continuous operation, the user hardwires the LBWSET pin
high and uses 1 μF capacitors to set the actual loop bandwidth.
These capacitors are placed between the PAVCAP and ERCAP pins
and ground. It is important that these capacitors are low leakage
multilayer ceramics with an insulation resistance greater than
100 GΩ or a time constant of 1000 seconds, whichever is less.
Setting LBSET low and using 47 nF capacitors results in a
shorter loop time constant (a 10× reduction over using 1 μF
capacitors and keeping LBWSET high).
Table 4.
Operation
Mode
Continuous
50 Mbps to
1.25 Gbps
Optimized for
1.25 Gbps
ALARMS
The ADN2848 is designed to allow interface compliance to
ITU-T-G958 (11/94), section 10.3.1.1.2 (transmitter fail) and
section 10.3.1.1.3 (transmitter degrade). The ADN2848 has two
active high alarms, DEGRADE and FAIL. A resistor between
ground and the ASET pin is used to set the current at which
these alarms are raised. The current through the ASET resistor
is a ratio of 100:1 to the FAIL alarm threshold. The DEGRADE
alarm is raised at 90% of this level.
AV
MPD_CW
CW
AV
is the average MPD current.
is the average power required.
is the dc optical power specified on the laser data sheet.
is the MPD current at that specified P
ERSET
and I
LBWSET
High
Low
PSET
change from device to device; however,
Recommended
PAVCAP
1 μF
47 nF
CW
.
Recommended
ERCAP
1 μF
47 nF
ADN2848