ADN2873(RevB) View Datasheet(PDF) - Analog Devices
Part Name
Description
MFG CO.
ADN2873 Datasheet PDF : 20 Pages
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ADN2873
Data Sheet
ALARMS
The ADN2873 has a latched, active high monitoring alarm
(FAIL). The FAIL alarm output is open drain in conformance
with SFP MSA specification requirements.
The ADN2873 has a three-fold alarm system that covers
• Use of a bias current that is higher than expected, likely as
a result of laser aging
• Out-of-bounds average voltage at the monitor photodiode
(MPD) input, indicating an excessive amount of laser
power or a broken loop
• Undervoltage in the IBIAS node (laser diode cathode) that
increases the laser power
The bias current alarm trip point is set by selecting the value
of resistor on the IBMON pin to GND. The alarm is triggered
when the voltage on the IBMON pin goes above 1.2 V. FAIL is
activated when the single-point alarms in Table 5 occur. The
circuit in Figure 37 can indicate that FAIL has been activated
while allowing the bias and modulation currents to remain on.
The VBE of the transistor clamps the FAIL voltage to below 1.3 V,
disabling the automatic shutdown of bias and modulation
currents. If an alarm has triggered and FAIL is activated, ALS
can be brought high and then low to clear the alarm.
VCC
R1
10kΩ
LED
D1
R2
330Ω
FAIL
Q1
NPN
ADN2873
Figure 37. FAIL Indication Circuit
Table 5. ADN2873 Single-Point Alarms
Alarm Type Mnemonic
Bias Current IBMON
MPD Current PAVSET
Crucial
Nodes
ERREF (the ERRREF designed is tied
to VCC in resistor setting mode)
IBIAS
Overvoltage or Short to VCC Condition
Alarm if >1.2 V typical (±10% tolerance)
Alarm if >2.0 V
Alarm if shorted to VCC (the alarm is
valid for voltage setting mode only)
Ignore
Undervoltage or Short to GND Condition
Ignore
Alarm if <0. 4 V
Ignore
Alarm if shorted to GND
Table 6. ADN2873 Response to Various Single-Point Faults in AC-Coupled Configuration (as shown in Figure 35 and Figure 36)
Pin
Short to VCC
Short to GND
Open
PAVSET Fault state occurs
Fault state occurs
Fault state occurs
PAVREF Voltage mode: fault state occurs
Fault state occurs
Fault state occurs
Resistor mode: tied to VCC
Circuit designed to tie to VCC in resistor
setting mode, so no open case
RPAV Voltage mode: fault state occurs
Fault state occurs
Voltage mode: fault state occurs
Resistor mode: tied to VCC
Resistor mode: does not increase
average power
PAVCAP Fault state occurs
Fault state occurs
Fault state occurs
DATAP Does not increase laser average power Does not increase laser average power
Does not increase laser average power
DATAN Does not increase laser average power Does not increase laser average power
Does not increase laser average power
ALS
Output currents shut off
Normal currents
Output currents shut off
ERSET Does not increase laser average power Does not increase laser average power
Does not increase laser average power
IMMON Does not affect laser power
Does not increase laser average power
Does not increase laser average power
ERREF Voltage mode: fault state occurs
Voltage mode: does not increase average Does not increase laser average power
power
Resistor mode: tied to VCC
Resistor mode: fault state occurs
IBMON Fault state occurs
Does not increase laser average power
Does not increase laser average power
FAIL
Fault state occurs
Does not increase laser average power
Does not increase laser average power
IMODP Does not increase laser average power Does not increase laser average power
Does not increase laser average power
IMODN Does not increase laser average power Does not increase laser average power
Does not increase laser power
IBIAS Fault state occurs
Fault state occurs
Fault state occurs
Rev. B | Page 18 of 20
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