ADN2847ACP-48 Просмотр технического описания (PDF) - Analog Devices
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ADN2847ACP-48 Datasheet PDF : 12 Pages
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ADN2847
Pin Number
48-Lead 32-Lead
34
35
36
37
38
25
39
26
40
26
41
27
42
28
43
28
44
29
45
30
46
31
47
31
48
32
PIN FUNCTION DESCRIPTIONS (continued)
Mnemonic
GND2
IDTONE
GND2
GND2
VCC2
IMODN
IMODN
GND2
IMODP
IMODP
GND2
GND2
IBIAS
IBIAS
CCBIAS
Function
Supply Ground
IDTONE (Requires External Current Sink to Ground)
Supply Ground
Supply Ground
Supply Voltage
Modulation Current Negative Output. Connect via a matching resistor to VCC.
Modulation Current Negative Output. Connect via a matching resistor to VCC.
Supply Ground
Modulation Current Positive Output. Connect to laser diode.
Modulation Current Positive Output. Connect to laser diode.
Supply Ground
Supply Ground
Laser Diode Bias Current
Laser Diode Bias Current
Extra Laser Diode Bias when AC-Coupled Current Sink
40
GENERAL
Laser diodes have current-in to light-out transfer functions as
shown in Figure 6. Two key characteristics of this transfer function
30
are the threshold current, ITH, and slope in the linear region
beyond the threshold current, referred to as slope efficiency, LI.
ER
=
P1
P0
20
P1
PAV
=
P1
+ P0
2
10
0
76 78 80 82 84 86 88 90 92 94 96 98 100
RISE TIME – ps
Figure 4. Rise Time Distribution Under Worst-
Case Operating Conditions
40
30
20
10
0
80 82 84 86 88 90 92 94 96 98 100 102 104
FALL TIME – ps
Figure 5. Fall Time Distribution Under Worst-
Case Operating Conditions
⌬P
PAV
⌬I
LI =
⌬P
⌬I
P0
ITH CURRENT
Figure 6. Laser Transfer Function
Control
A monitor photodiode, MPD, is required to control the LD.
The MPD current is fed into the ADN2847 to control the 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 slope efficiency.
The ADN2847 uses automatic power control, APC, to maintain
a constant average power over time and temperature.
The ADN2847 uses closed-loop extinction ratio control to allow
optimum setting of extinction ratio for every device. Thus
SONET/SDH interface standards can be met over device
variation, temperature, and laser aging. 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 RPSET is used to
change the average power. The potentiometer RERSET is used to
adjust the extinction ratio. Both PSET and ERSET are kept
1.2 V above GND.
–6–
REV. 0
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