DS1886 datasheet(22/92 Pages) MAXIM

22 / 92 page

DS1886

SFP and PON ONU Controller

with Digital LDD Interface

Maxim Integrated

Laser Bias and Laser Power Through TXMON

The DS1886 measures both laser bias (TXB) and laser

power (TXP) through the same input pin, TXMON. The

DS1886 commands the MAX3710 laser driver to output

the correct monitor signal before each ADC conversions

takes place. Figure 4 shows the two conversion paths.

Each path has independent gain and offset calibration

registers.

Enhanced RSSI Monitoring

(Dual Range Functionality)

The DS1886 offers a feature to improve the accuracy

and range of RSSI, which is most commonly used for

monitoring RSSI. To achieve the SFF-8472 requirement of

0.1µW/LSB over -40 to 8.2dBm, the DS1886 makes two

measurements to effectively achieve a 16-bit conversion

with a 13-bit physical ADC. This “dual range” calibration

can operate in two modes: APD mode and PIN mode.

APD Mode

For systems with a nonlinear relationship between the ADC

input and desired ADC result, the mode should be set to

APD mode (Figure 5). The RSSI measurement of an APD

receiver is one such application. Using the APD mode

allows a piece-wise linear approximation of the nonlinear

response of the APD’s gain factor. The crossover point is

the point between fine and coarse points. The ADC result

transitions between the fine and coarse ranges with no

hysteresis. Right-shifting, slope adjustment, and offset

are configurable for both the fine and coarse ranges. Two

registers, XOVER FINE and XOVER COARSE, determine

the crossover point. The XOVER FINE register (A2h Table

02h, Register A0h–A1h) determines the maximum results

returned by fine ADC conversions, before right-shifting.

The XOVER COARSE register (A2h Table 02h, Register

90h–91h) determines the minimum results returned by

coarse ADC conversions, before right-shifting.

Figure 5. RSSI in APD Mode

Figure 4. Laser Bias (TXB) and Laser Power (TXP) Monitoring Through TXMON

CROSSOVER POINT

RSSI RESULT

APD MODE

IDEAL RESPONSE

FINE FULL-SCALE

RESPONSE

RSSI INPUT

TXMON

BMON

MON_SEL = 0

TXB

TXP

ADC

DS1886

MAX3710

TXMON

BMON

MON_SEL = 1

TXB

TXP

ADC

DS1886

MAX3710

DS1886 Datasheet(PDF) 22 Page - Maxim Integrated Products

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