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參數資料
| 型號： | ADN2841ACP-48 |
| 廠商： | ANALOG DEVICES INC |
| 元件分類： | 其它接口 |
| 英文描述： | Dual-Loop 50 Mbps.2.7 Gbps Laser Diode Driver |
| 中文描述： | SPECIALTY INTERFACE CIRCUIT, QCC48 |
| 封裝： | MO-220-VKKD-2, LFCSP-48 |
| 文件頁數： | 6/12頁 |
| 文件大小： | 276K |
| 代理商： | ADN2841ACP-48 |
REV. 0
ADN2841
–6–
LOOP BANDWIDTH SELECTION
For anyrate operation the user should hardwire the LBWSET
pin high and use 1
μ
F capacitors to set the actual loop band-
width. These capacitors are placed between the PAVCAP and
ERCAP pins and ground. It is important that these capacitors
be low-leakage multilayer ceramics with an insulation resistance
greater than 100 G
or a time constant of 1000 sec, whichever
is less. The ADN2841 may be optimized for 2.7 Gbps operation
by keeping the LBWSET pin low. This results in a much shorter
loop time constant (a 10 reduction). The value of PAVCAP
and ERCAP capacitors required for 2.5 Gbps operation is 22 nF.
ALARMS
The ADN2841 alarms are 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 ADN2841 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.
Example:
=
∴
50
I
mA
I
mA
FAIL
DEGRADE
=
45
I
I
mA
A
ASET
BIASTRIP
100
=
=
=
μ
50
100
500
R
V
I
V
μ
A
k
ASET
ASET
=
=
=
1 23
.
1 23
500
2 46
.
.
NOTE: The smallest value for R
ASET
is 1.2 k
, as this corre-
sponds to the I
BIAS
maximum of 100 mA.
The laser degrade alarm, DEGRADE, gives a warning of imminent
laser failure if the laser diode degrades further or environmental
conditions continue to stress the LD, e.g., increasing temperature.
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:
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 in which
ALS has been enabled.
DEGRADE will only be raised when the bias current exceeds
90% of ASET current.
MONITOR CURRENTS
IBMON, IMMON, and IMPDMON and IMPDMON2 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 pro-
portional to the current monitored.
DUAL MPD DWDM FUNCTION (48-PIN LFCSP ONLY)
The ADN2841 has circuitry for an optional second monitor
photodiode, MPD2.
GENERAL
Laser diodes have current-in to light-out transfer functions as shown
in Figure 2. Two key characteristics of this transfer function are the
threshold current, I
TH
, and slope in the linear region beyond the
threshold current, referred to as slope efficiency, LI.
ER =
P
AV
=
P1
P0
P1 + P0
2
P
I
LI =
P
I
I
TH
CURRENT
P1
P
AV
P0
O
Figure 2. Laser Transfer Function
CONTROL
A monitor photodiode (MPD) is required to control the LD. The
MPD current is fed into the ADN2841 to control the optical
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 (LI) slope (slope efficiency).
The ADN2841 uses automatic power control (APC) to main-
tain a constant power over time and temperature.
The ADN2841 uses closed-loop extinction ratio control to allow
optimum setting of extinction ratio for every device. Hence
SONET/SDH interface standards can be met over device varia-
tion, temperature, and time. 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
PSET
, is used to
change the average power. The potentiometer,
R
ERSET
, is used
to adjust the extinction ratio. Both PSET and ERSET are
kept 1.23 V above GND.
R
PSET
and R
ERSET
can be calculated using the following formulas:
R
V
I
PSET
AV
=
1 23
.
where
I
AV
is average MPD current.
R
V
+
I
P
ER
ER
P
ERSET
MPD
CW
CW
AV
=
×
×
×
1 23
.
1
1
0 2
.
_
where
P
CW
is the dc optical power specified on the laser data
sheet,
I
MPD_CW
is MPD current at that specified
P
CW
, and
P
AV
is
the required average power.
Note that I
ERSET
and I
PSET
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.
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