DS1874 datasheet(13/88 Pages) MAXIM

13 / 88 page

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).

______________________________________________________________________________________

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

Burst Mode

DAC

Digital-to-Analog Converter

LOS

Loss of Signal

LUT

Lookup Table

Nonvolatile

Quick Trip

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

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)

Figure 1. Modulation LUT Loading to MAX3798/MAX3799 MOD DAC

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