Datasheet

DS1886
SFP and PON ONU Controller
with Digital LDD Interface
22Maxim 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
COARSE FULL-SCALE RESPONSE
RSSI INPUT
TXMON
BMON
MON_SEL = 0
TXB
TXP
ADC
ADC
DS1886
MAX3710
TXMON
BMON
MON_SEL = 1
TXB
TXP
ADC
ADC
DS1886
MAX3710