ADN2850 Analog Devices, Inc., ADN2850 Datasheet - Page 16

• Manufacturer Part Number: ADN2850
• Description: Nonvolatile Memory, Dual 1024 Position Programmable Resistors
• Manufacturer: Analog Devices, Inc.

ADN2850
APPLICATIONS
Optical Transmitter Calibration with ADN2841
Together with the multirate 2.7 Gbps Laser Diode Driver ADN2841,
the ADN2850 forms an optical supervisory system where the dual
programmable resistors are used to set the laser average optical
power and extinction ratio (see Figure 11). The ADN2850 is
particularly ideal for the optical parameter settings because of its
high resolution, compact footprint, and superior temperature
coefficient characteristics.
The ADN2841 is a 2.7 Gbps laser diode driver that uses a unique
control algorithm to manage both the laser average power and
extinction ratio after the laser initial factory calibration. It stabilizes
the laser data transmission by continuously monitoring its optical
power, and correcting the variations caused by temperature and
the laser degradation over time. In the ADN2841, the I
the laser diode current. Through its dual-loop power and extinction
ratio control, calibrated by the ADN2850, the internal driver
controls the bias current I
It also regulates the modulation current I
modulation current linearly with slope efficiency. Any changes in
the laser threshold current or slope efficiency are therefore com-
pensated. As a result, this optical supervisory system minimizes the
laser characterization efforts and enables designers to apply com-
parable lasers from multiple sources.
Incoming Optical Power Monitoring
The ADN2850 comes with a pair of matched diode connected
PNPs, Q
power monitoring function. With a reference current source, an
instrumentation amplifier, and a logarithmic amplifier, this feature
can be used to monitor the optical power by knowing the dc
average photodiode current from the following relationships:
V = V
V = V
1
2
1
and Q
BE1
BE2
= V
= V I
2
, that can be used to configure an incoming optical
T
T
In
n
I
I
I
I
C
S
C
S
1
1
2
2
BIAS
and consequently the average power.
Figure 12. Conceptual Incoming Optical Power Monitoring Circuit
V
V
SS
DD
B
ADN2850
1
W
MODP
1
B
2
by changing the
W
2
MPD
–5V
Q
1
GND
V
10nF
I
1
PD
monitors
Q
(2)
(3)
2
TIA
V
I
R
2
REF
–16–
G
LOG AMP
0.75 BIT RATE
IN AMP
Knowing I
therefore a and I
theoretically yields:
Where I
V
transistors
V
k = Boltzmann’s constant = 1.38E–23 Joules/Kelvin
q = electron charge = 1.6E–19 coulomb
T = temperature in Kelvin
I
I
Figure 12 shows such a conceptual circuit.
CLK
AD623
SDI
PD
REF
CS
1
T
V
, V
LPF
V – V = V In
is the thermal voltage, which is equal to k × T/q.
T
= photodiode current
(1 + 100k/R
= reference current
2
= 26 mV at 25°C
2
ADN2850
CONTROL
are V
S1
1
C1
and I
Figure 11. Optical Supervisory System
POST
VT COMPENSATION
AMP
G
BE
= a
)
, base-emitted voltages of the diode connector
T
PRC
THERMISTOR
C
S2
(V
1
S
EEMEM
EEMEM
2
are matched. Combining Equations 2 and 3
are saturation current
CDR
– V



I
I
PD,
I
1
REF
)
PD
I
C2



RDAC1
RDAC2
DATA
CLOCK
= a
LOG
AVERAGE
POWER
2
IDTONE
DINQ
W1
W2
B1
B2
DIN
I
REF,
and Q
PSET
ERSET
ADN2841
1
– Q
I MPD
I
MODP
I
BIAS
2
V
CC
are matched,
REV. B
V
CC
(4)