ADN2841 Analog Devices, Inc., ADN2841 Datasheet - Page 7

• Manufacturer Part Number: ADN2841
• Description: Dual-loop 50 Mbps.2.7 Gbps Laser Diode Driver
• Manufacturer: Analog Devices, Inc.

The second photodiode current is mirrored to IMPDMON2
for wavelength control purposes and is summed internally for
the power control loop. For single MPD circuits, the MPD2 pin
is tied to GND.
This enables the system designer to use the two currents to
control the wavelength of the laser diode using various optical
filtering techniques inside the laser module.
If the monitor current functions, IMPDMON and IMPDMON2
are not required, the IMPD and IMPD2 pins can be grounded,
and the monitor photodiode output can be connected directly
to PSET.
IDTONE (48-LEAD LFCSP ONLY)
The IDTONE pin is supplied for fiber identification/supervisory
channels or control purposes in WDM. This pin modulates the
optical one level over a possible range of 2% of minimum IMOD
to 10% of maximum IMOD. The level of modulation is set by
connecting an external current sink between the IDTONE pin and
ground. There is a gain of two from this pin to the IMOD current.
Figure 3 shows how an AD9850/AD9851 DDS may be used
with the ADN2841 to allow fiber identification.
Note that using IDTONE during transmission may cause opti-
cal eye degradation.
DATA, CLOCK INPUTS
Data and clock inputs are ac-coupled (10 nF recommended)
and terminated via a 100 Ω internal resistor between DATAP
and DATAN and also between CLKP and CLKN pins. There
is a high impedance circuit to set the common-mode voltage
that is designed to change overtemperature. It is recommended
that ac-coupling be used to eliminate the need for matching
between common-mode voltages.
REV. A
CLKIN
AD9850/AD9851
REF CLOCK
20MHz–
180MHz
CONTROLLER
R
DDS
SET
37.5 A–600 A
I
I
OUT
OUT
50
1.25mA–20mA
Figure 3. Circuitry to Allow Fiber Identification
50
(DC-COUPLED)
LP FILTER
1300
BC550
–7–
CCBIAS
CCBIAS should be connected to the BIAS pin if the laser diode is
connected to the ADN2841 using a capacitor. CCBIAS is a
current sink to GND.
AUTOMATIC LASER SHUTDOWN
The ADN2841 ALS allows compliance to ITU-T-G958
(11/94), section 9.7.
When ALS is logic high, both bias and modulation currents are
turned off.
Correct operation of ALS can be confirmed by the fail alarm
being raised when ALS is asserted. Note this is the only time
that DEGRADE will be low while FAIL is high.
AD8602
1/2
AD8602
1/2
Figure 4. AC-Coupling of Data Inputs
BC550
500
DATAP
DATAN
10kHz–1MHz
0.125mA–2mA
50 A–800 A
50
1000
ADN2841
50
TO FLIP-FLOPS
V
REG
400 A TYP
R
R = 2.5k , DATA
R = 3k , CLK
ADN2841
IMMON
IDTONE
ADN2841