ADN2847
The RPSET and RERSET potentiometers can be calculated using the
The laser degrade alarm, DEGRADE, is provided to give a warning
of imminent laser failure if the laser diode degrades further or
environmental conditions continue to stress the LD, such as
increasing temperature.
following formulas.
1.2V
I AV
RPSET
=
Ω
( )
The laser fail alarm, FAIL, is activated when the transmitter can no
longer be guaranteed to be SONET/SDH compliant. This occurs
when one of the following conditions arises:
1.2V
RERSET
=
Ω
( )
IMPD _ CW
ER − 1
ER + 1
×
× PAV
PCW
•
•
The ASET threshold is reached.
where:
I
P
I
The ALS pin is set high. This shuts off the modulation and bias
currents to the LD, resulting in the MPD current dropping
to zero. This gives closed-loop feedback to the system that
ALS has been enabled.
AV is the average MPD current.
CW is the dc optical power specified on the laser data sheet.
MPD_CW is the MPD current at that specified PCW.
AV is the average power required.
P
DEGRADE will be raised only when the bias current exceeds
90% of ASET current.
ER is the desired extinction ratio (ER = P1/P0).
Note that IERSET and IPSET will change from device to device;
however, the control loops will determine actual values. It is not
required to know exact values for LI or MPD optical coupling.
Monitor Currents
IBMON, IMMON, IMPDMON, and IMPDMON2 are current
controlled current sources from VCC. They mirror the bias, modu-
lation, and MPD current for increased monitoring functionality.
An external resistor to GND gives a voltage proportional to the
current monitored.
Loop Bandwidth Selection
For continuous operation, the user should hardwire the LBWSET
pin high and use 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 sec, whichever is less.
If the monitoring functions IMPDMON and IMPDMON2 are
not required, the IMPD and IMPD2 pins must be grounded
and the monitor photodiode output must be connected directly
to the PSET pin.
Operation
Mode
Recommended Recommended
Dual MPD DWDM Function (48-Lead LFCSP Only)
The ADN2847 has circuitry for a second monitor photodiode,
MPD2. The second photodiode current is mirrored to IMPDMON2
for wavelength control purposes and is summed internally with
the first monitor photodiode current for the power control loop.
For single MPD circuits, the MPD2 pin is tied to GND.
LBWSET PAVCAP
ERCAP
Continuous
50 Mbps to
3.3 Gbps
Optimized
for 2.5 Gbps
to 3.3 Gbps
High
1 µF
1 µF
Low
22 nF
22 nF
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.
Setting LBSET low and using 22 nF capacitors results in a
shorter loop time constant (a 10× reduction over using 1 µF
capacitors and keeping LBWSET high.)
If the monitor current functions IMPDMON and IMPDMON2
are not required, then the IMPD and IMPD2 pins can be
grounded and the monitor photodiode output can be connected
directly to PSET.
Alarms
The ADN2847 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 ADN2847 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 will be raised at 90% of this level.
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 9 shows how an AD9850/AD9851 or the AD9834 may be
used with the ADN2847 to allow fiber identification.
Example:
IFAIL = 50 mA so IDEGRADE = 45 mA
If the ID_TONE function is not used, the IDTONE pin should
be tied to VCC. Note that using IDTONE during transmission
may cause optical eye degradation.
IFAIL 50 mA
IASET
=
=
=
= 500 µA
100
100
Data, Clock Inputs
1.2V
IASET 500 ꢀA
1.2
*
Data and clock inputs are ac-coupled (10 nF capacitors are
recommended) and terminated via a 100 Ω internal resistor between
DATAP and DATAN, and also between the CLKP and CLKN
pins. There is a high impedance circuit to set the common-mode
voltage that is designed to allow for maximum input voltage
RASET
=
= 2.4 kΩ
* The smallest valid value for RASET is 1.2 kΩ, since this corresponds to the IBIAS
maximum of 100 mA.
REV. 0
–7–
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