ADN2843 Analog Devices, ADN2843 Datasheet - Page 4

• Manufacturer Part Number: ADN2843
• Description: 10.7 Gbps, 3.3V Laser Diode Driver With Dual-loop Control
• Manufacturer: Analog Devices

GENERAL
CONTROL
control the LD. The MPD current is fed into the
ADN2843 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, LI, slope (slope efficiency).
APC, to maintain a constant power over time
and temperature.
ratio control to allow optimum setting of
extinction ratio for every device. Hence
SONET/SDH interface standards can be met
over device variation, temperature and time.
Closed loop modulation control eliminates the
need to either over modulate the laser diode or
include external components for temperature
compensation, thus reducing R&D and second
sourcing issues.
PSET and ERSET pins respectively. A resistor
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 and the slope in the linear
region beyond the threshold current, referred
to as slope efficiency, LI.
Figure 2: Laser Transfer Function
A monitor photo diode, MPD, is required to
The ADN2843 uses automatic power control,
The ADN2843 uses closed loop extinction
Power and Extinction Ratio are set using the
ADN2843
Pav
P1
Po
ER = P1
Pav=P1+Po
Po
2
PRELIMINARY TECHNICAL DATA
Ith
I
Current
P
LI = P
I
- 4 -
laser datasheet at which I
laser diode to laser diode, therefore R
R
When tuning the Laser Diode, R
adjusted first with R
average power is set, R
the desired extinction ratio. Once the values
R
desired average power and extinction ratio, the
control loops maintain these values of average
power and extinction ratio over environmental
conditions and time.
LOOP TIME-CONSTANT SELECTION
is placed between the pin and GND to set the
current flowing in each pin. The internal control
loops force the PSET and ERSET pins to 1.23V
above GND.
given by the following formulae:
The ratios of current-in-to-current-out are
P
The control loop time constant can be
optimised for operation at data-rates of
2.5GB/s and above by setting the LBWSET
pin low. This results in a faster loop time
constant. The required value for the
PAVCAP/ERCAP capacitors is 22nF in this
case.
PSET
PSET
Note that I
CW
I
AV
need to be adjusted for each Laser Diode .
is the DC optical power specified on the
and R
is the average MPD current.
R
ERSET
ERSET
ERSET
have been adjusted to set the
and I
R
ERSET
PSET
I
MPD
P
ERSET
PSET
CW
MPD_CW
_
at 25k . Once the
CW
14-Jan-2002 REV. PrA
. 1
. 1
I
will change from
AV
23
is adjusted to set
23
ER
ER
PSET
is specified.
should be
1
1
ERSET
and