ADN2847
headroom over temperature. It is necessary that ac coupling is
used to eliminate the need for matching between common-
mode voltages.
currents are turned off. Correct operation of ALS can be
confirmed by the FAIL alarm being raised when ALS is as-
serted. Note that this is the only time DEGRADE will be low
while FAIL is high.
Alarm Interfaces
ADN2847
The FAIL and DEGRADE outputs have an internal pull-up
DATAP
(TO FLIP-FLOPS)
DATAN
resistor of 30 kΩ used to pull the digital high value to VCC
.
However, the alarm can be overdriven with an external resistor
allowing alarm interfacing to non-VCC levels. Non-VCC alarm
output levels must be below the VCC used for the ADN2847.
50ꢄ 50ꢄ
V
REG
R
Power Consumption
The ADN2847 die temperature must be kept below 125oC. Both
LFCSP packages have an exposed paddle, which should be
connected in such a manner that is is at the same potential as
the ADN2847 ground pins. The θJA for both packages is shown
in the Absolute Maximum Ratings. Power consumption can be
calculated using
R = 2.5kꢄ, DATA
R = 3kꢄ, CLK
400ꢀATYP
Figure 7. AC Coupling of Data Inputs
ICC = ICCMIN + 0.3 IMOD
For input signals that exceed 500 mV p-p single ended, it is
necessary to insert an attenuation circuit as shown in Figure 8.
P =VCC × ICC + I
×VBIAS _ PIN + I
V
(
+VMODN _ PIN / 2
(
)
)
BIAS
MOD
MODP _ PIN
TDIE = TAMBIENT + θJA × P
ADN2847
Thus, the maximum combination of IBIAS + IMOD must be calcu-
lated. Where:
R1
R2
DATAP/CLKP
DATAN/CLKN
R
R3
IN
ICCMIN
= 50 mA, the typical value of ICC provided on page 3
with IBIAS = IMOD = 0
T
T
DIE = die temperature
NOTETHAT R = 100ꢄ =THE DIFFERENTIAL
INPUT IMPEDANCE OFTHE ADN2847
IN
AMBIENT = ambient temperature
V
V
V
BIAS_PIN = voltage at IBIAS pin
Figure 8. Attenuation Circuit
MODP_PIN = average voltage at IMODP pin
MODN_PIN = average voltage at IMODN pin
CCBIAS
When the laser is used in ac-coupled mode, the CCBIAS and
IBIAS pins should be tied together (Figure 12). In dc-coupled
Laser Diode Interfacing
mode, CCBIAS should be tied to VCC
.
Many laser diodes designed for 2.5 Gbs operation are packaged
with an internal resistor to bring the effective impedance up to
25 Ω in order to minimize transmission line effects. In high current
applications, the voltage drop across this resistor combined with
the laser diode forward voltage makes direct connection between
the laser and the driver impractical in a 3 V system. AC coupling
the driver to the laser diode removes this headroom constraint.
Automatic Laser Shutdown
The ADN2847 ALS allows compliance to ITU-T-G958 (11/94),
section 9.7. When ALS is logic high, both bias and modulation
REF CLOCK
20MHz–180MHz
10kHz–1MHz
1/2
35
IDTONE
CLKIN
AD8602
9
1.25mA–20mA
0.125mA–2mA
BC550
LP FILTER
(DC-COUPLED)
21
I
OUT
AD9850/AD9851
AD9834
50ꢄ
ADN2847
DDS
20
I
OUT
12
500ꢄ
R
50ꢄ
SET
1/2
AD8602
37.5ꢀA–600ꢀA
50ꢀA–800ꢀA
CONTROLLER
32
IMMON
BC550
1000ꢄ
1300ꢄ
Figure 9. Application Curcuit to Allow Fiber Identification Using the AD9850/AD9851
–8–
REV. 0
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