SLAAEF1 august   2023 AFE11612-SEP

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
  5. 2Laser and Semiconductor Optical Amplifier Biasing
  6. 3EML Negative Bias Voltage
  7. 4Photodiode Detection Measurement
  8. 5Variable Optical Attenuator Control
  9. 6Summary
  10. 7References

4 Photodiode Detection Measurement

The AFE11612-SEP has 16 ADC inputs that can be used for monitoring many functions, including measuring photodiode current. The device ADC is a successive-approximation register (SAR) ADC. SAR ADCs have an internal sampling capacitor which must be charged every time there is an ADC conversion. This capacitor must be charged within the sample acquisition time to ensure the ADC measures the voltage correctly. This is done using a charge-bucket filter with an external capacitor (CFILT). The filter resistor (RFILT) can be a low value so CFILT can receive enough current to be charged for each sample. A sense resistor (RSENSE) turns the photodiode current into a voltage, and the resistor should be selected such that the resulting voltage is within the ADC 5-V input range. Figure 4-1 shows the charge bucket circuit.

GUID-20230727-SS0I-GMGG-M21P-CHRMQDJF5WMH-low.svg Figure 4-1 Photodiode Monitor Circuit