ADN2847ACPZ-32 데이터 시트보기 (PDF) - Analog Devices
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ADN2847ACPZ-32 Datasheet PDF : 16 Pages
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Data Sheet
THEORY OF OPERATION
Laser diodes have current-in to light-out transfer functions, as
shown in Figure 9. Two key characteristics of this transfer
function are the threshold current, ITH, and the slope in the
linear region beyond the threshold current, referred to as slope
efficiency (LI).
ER
=
P1
P0
P1
PAV
=
P1
+
2
P0
∆P
PAV
LI = ∆P
∆I
∆I
P0
ITH CURRENT
Figure 9. 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 changing threshold
current and light-to-current slope efficiency of the laser.
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 the 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 pin
and the ERSET pin, 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.
The RPSET and RERSET potentiometers can be calculated using the
following formulas:
RPSET
=
1.2 V
I AV
(Ω)
RERSET
=
I MPD _ CW
PCW
1.2 V
×
ER
ER
−1
+1
× PAV
(Ω)
ADN2847
where:
IAV is the average MPD current.
PCW is the dc optical power specified on the laser data sheet.
IMPD_CW is the MPD current at that specified PCW.
PAV is the average power required.
ER is the desired extinction ratio (ER = P1/P0).
Note that IERSET and IPSET changes from device to device; however,
the control loops determines actual values. It is not required to
know exact values for LI or MPD optical coupling.
LOOP BANDWIDTH SELECTION
For continuous operation, the user should hardwire the LBWSET
pin high and use 1 μF capacitors to set the actual loop bandwidth.
These capacitors are placed between the PAVCAP pin and the
ERCAP pin and ground. It is important that these capacitors are
low leakage multilayer ceramics with an insulation resistance
greater than 100 GΩ or a time constant of 1000 seconds,
whichever is less.
Table 6.
Operation
Mode
Continuous
50 Mbps to
3.3 Gbps
Optimized for
2.5 Gbps to
3.3 Gbps
Recommended Recommended
LBWSET PAVCAP
ERCAP
High
1 μF
1 μF
Low
22 nF
22 nF
Setting LBSET low and using 22 nF capacitors results in a
shorter loop time constant (a 10× reduction over using 1 μF
capacitors and keeping LBWSET high.)
ALARMS
The ADN2847 is designed to allow interface compliance to
ITUT-G958 (11/94) section 10.3.1.1.2 (transmitter fail) and
section 10.3.1.1.3 (transmitter degrade). The ADN2847 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 is raised at 90% of this level.
Example:
I FAIL = 50 mA so I DEGRADE = 45 mA
I ASET
=
I FAIL
100
=
50 mA
100
= 500 μA
R ASET
=
1.2 V
I ASET
=
1.2
500 μA
= 2.4
kΩ
where the smallest valid value for RASET is 1.2 kΩ, because this
corresponds to the IBIAS maximum of 100 mA.
Rev. B | Page 9 of 16
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