SBOA601 January   2025 LOG200

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
  5. 2Critical Photodiode Specifications
  6. 3Interfacing the LOG200 With the Photosensor
    1. 3.1 Photodiode Connections
    2. 3.2 Photodiode Adaptive Biasing Current Output
  7. 4Optical Bench for Current Sensing Measurements
    1. 4.1 Transient Response with Photosensor
  8. 5Optical Power Measurements with the LOG200
  9. 6Error Sources and Uncalibrated Error Analysis
  10. 7Auxiliary Op Amp Circuits
    1. 7.1 Single-Ended to Differential Conversion Circuit
    2. 7.2 Sallen-Key Low-Pass Filter
  11. 8Summary
  12. 9References

3.1 Photodiode Connections

The LOG200EVM PCB board layout includes a photosensor footprint (D1) to install a radial photodiode on current input I1 for the logarithmic numerator and optional passive component footprints for compensation and photosensor biasing circuits. Refer to Figure 3-1 for the LOG200EVM photosensor biasing connections.

The LOG200 photodiode IBIAS adaptive biasing current output is accessible through an optional jumper resistor R2. The adaptive current output biasing scheme creates a voltage to bias a photodiode with a current proportional to the photocurrent. Compensation capacitor C1 (CCOMP) helps providing dynamic currents during fast transients to improve stability. The following section discusses the current adaptive biasing feature in detail.

Alternatively, the photosensor cathode can be reverse-biased via test point TP1, DBIAS. When not using the photodiode adaptive biasing feature, leave the IBIAS pin floating. Remove resistor R2 when biasing the photosensor through TP1.