Rainbow Electronics DS1874 - Manual (Page 13)

an external resistor. The feedback is sampled by a com-
parator and compared to a digital set-point value. The
output of the comparator has three states: up, down, or
no-operation. The no-operation state prevents the output
from excessive toggling once steady state is reached.
As long as the comparator output is in either the up or
down states, the bias is adjusted by writing increment
and decrement values to the MAX3798/MAX3799
through the BIASINC register (3-wire address 13h).
The DS1874 has an LUT to allow the APC set point to
change as a function of temperature to compensate for
tracking error (TE). The TE LUT has 36 entries that
determine the APC setting in 4°C windows between
-40°C to +100°C.
MODULATION Control
The MAX3798/MAX3799 control the laser modulation
using the internal temperature-indexed LUT within the
DS1874. The modulation LUT is programmed in 2°C
increments over the -40°C to +102°C range to provide
temperature compensation for the laser’s modulation.
The modulation is updated after each temperature con-
version using the 3-wire interface that connects to the
MAX3798/MAX3799. The MAX3798/MAX3799 include a
9-bit DAC. The modulation LUT is 8 bits.
Figure 1 demonstrates how the 8-bit LUT controls the
9-bit DAC with the use of a temperature control bit
(MODTC, Table 02h, Register C6h) and a temperature
index register (MODTI, Table 02h, Register C2h).
DS1874
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Table 1. Acronyms
ACRONYM DEFINITION
ADC Analog-to-Digital Converter
AGC Automatic Gain Control
APC Automatic Power Control
APD Avalanche Photodiode
ATB Alarm Trap Bytes
BM Burst Mode
DAC Digital-to-Analog Converter
LOS Loss of Signal
LUT Lookup Table
NV Nonvolatile
QT Quick Trip
TE Tracking Error
TIA Transimpedance Amplifier
ROSA Receiver Optical Subassembly
SEE Shadowed EEPROM
SFF Small Form Factor
SFF-8472
Document Defining Register Map of SFPs
and SFFs
SFP Small Form Factor Pluggable
SFP+ Enhanced SFP
TOSA Transmit Optical Subassembly
TXP Transmit Power
SFP+ Controller with Digital LDD Interface
MOD LUT
LOADED TO [7:0]
MOD LUT
LOADED TO [7:0]
MODTI
8
7
6
5
4
3
2
1
0
MODTI
MODTC = 0
TEMPERATURE ( °C)
-40 +102
TEMPERATURE ( °C)
-40 +102
MODTC = 1
MOD LUT
LOADED TO [8:1]
(DAC BIT 0 = 0)
MOD LUT
LOADED TO [8:1]
(DAC BIT 0 = 0)
MAX3798/MAX3799 DAC BIT
8
7
6
5
4
3
2
1
0
MAX3798/MAX3799 DAC BIT
Figure 1. Modulation LUT Loading to MAX3798/MAX3799 MOD DAC