Datasheet

ADN2848 Data Sheet
Rev. B | Page 8 of 12
Example:
mA45mA
50 =
=
DEGRADE
FAIL
Iso
I
μA500
100
mA50
100
===
FAIL
ASET
I
I
kΩ4.2
μA500
2.1V2.1
* ===
ASET
ASET
I
R
*The smallest valid value for R
ASET
is 1.2 kΩ, because this corresponds to the
maximum I
BIAS
of 100 mA.
The laser degrade alarm, DEGRADE, is provided to give a
warning of imminent laser failure if the laser diode degrades
further or if environmental conditions such as increasing
temperature continue to stress the LD.
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 arise:
• The ASET threshold is reached.
• 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.
DEGRADE is raised only when the bias current exceeds 90% of
ASET current.
MONITOR CURRENTS
IBMON, IMMON, and IMPDMON are current controlled
current sources from V
CC
. They mirror the bias, modulation,
and MPD current for increased monitoring functionality. An
external resistor to GND gives a voltage proportional to the
current monitored.
If the monitoring function IMPDMON is not required, the
IMPD pin must be grounded and the monitor photodiode
output must be connected directly to the PSET pin.
DATA AND CLOCK INPUTS
Data and clock inputs are ac-coupled (10 nF capacitors
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, which is designed to allow for
maximum input voltage headroom over temperature. It is
necessary that ac coupling be used to eliminate the need for
matching between common-mode voltages.
ADN2848
TO FLIP-FLOPS
50Ω 50Ω
V
REG
R
R = 2.5kΩ, DATA
R = 3kΩ, CLK
400µA TYP
DATAP
DATAN
02746-005
Figure 5. AC Coupling of Data Inputs
For input signals that exceed 500 mV p-p single-ended, it is
necessary to insert an attenuation circuit as shown in Figure 6.
02746-006
DATAP/CLKP
DATAN/CLKN
R3
R1
R2
R
IN
ADN2848
NOTE THAT R
IN
= 100Ω = THE DIFFERENTIAL
INPUT IMPEDANCE OF THE ADN2848.
Figure 6. Attenuation Circuit
CCBIAS
When the laser is used in ac-coupled mode, the CCBIAS pin
and the I
BIAS
pin are tied together (see Figure 9). In dc-coupled
mode, CCBIAS is tied to V
CC
.
I
BIAS
To achieve optimum optical eye quality, a pull-up resistor R
Z
, as
shown in Figure 8 and Figure 9, is required. The recommended
R
Z
value is approximately 200 Ω ~ 500 Ω.
AUTOMATIC LASER SHUTDOWN
The ADN2848 ALS allows compliance to ITU-T-G958 (11/94),
section 9.7. When ALS is logic high, both the bias and the
modulation currents are turned off. Correct operation of ALS is
confirmed by the FAIL alarm being raised when ALS is
asserted. Note that this is the only time that DEGRADE is low
while FAIL is high.
ALARM INTERFACES
The FAIL and DEGRADE outputs have an internal 30 kΩ pull-
up resistor that is used to pull the digital high value to V
CC
.
However, the alarm output can be overdriven with an external
resistor, allowing alarm interfacing to non-V
CC
levels. Non-V
CC
alarm output levels must be below the V
CC
used for the
ADN2848.