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ADN2847 Datasheet(PDF) 6 Page - Analog Devices

Part No. ADN2847
Description  3 V Dual-Loop 50 Mbps to 3.3 Gbps Laser Diode Driver
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com
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ADN2847 Datasheet(HTML) 6 Page - Analog Devices

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REV. 0
–6–
ADN2847
RISE TIME – ps
0
76
10
20
30
40
78
80
82
84
86
88
90
92
94
96
98
100
Figure 4. Rise Time Distribution Under Worst-
Case Operating Conditions
FALL TIME – ps
0
80
10
20
30
40
82
84
86
88
90
92
94
96
98
100 102 104
Figure 5. Fall Time Distribution Under Worst-
Case Operating Conditions
GENERAL
Laser diodes have current-in to light-out transfer functions as
shown in Figure 6. Two key characteristics of this transfer function
are the threshold current, ITH, and slope in the linear region
beyond the threshold current, referred to as slope efficiency, LI.
ER =
PAV =
P1
P0
P1 + P0
2
P
I
LI =
P
I
ITH
CURRENT
P1
PAV
P0
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 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
1.2 V above GND.
PIN FUNCTION DESCRIPTIONS (continued)
Pin Number
48-Lead
32-Lead
Mnemonic
Function
34
GND2
Supply Ground
35
IDTONE
IDTONE (Requires External Current Sink to Ground)
36
GND2
Supply Ground
37
GND2
Supply Ground
38
25
VCC2
Supply Voltage
39
26
IMODN
Modulation Current Negative Output. Connect via a matching resistor to VCC.
40
26
IMODN
Modulation Current Negative Output. Connect via a matching resistor to VCC.
41
27
GND2
Supply Ground
42
28
IMODP
Modulation Current Positive Output. Connect to laser diode.
43
28
IMODP
Modulation Current Positive Output. Connect to laser diode.
44
29
GND2
Supply Ground
45
30
GND2
Supply Ground
46
31
IBIAS
Laser Diode Bias Current
47
31
IBIAS
Laser Diode Bias Current
48
32
CCBIAS
Extra Laser Diode Bias when AC-Coupled Current Sink


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