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

5 Optical Power Measurements with the LOG200

A photodiode detector is a p-n junction semiconductor that can generate current if the energy of incident photons exceeds the material's bandgap. The sensitivity or responsivity of a photodiode (R) can be defined as the ratio of the generated photocurrent (IPD) to the incident light power (PPD) at a given wavelength (ƛ). Equation 5 show the relation of the photodiode responsivity:

Equation 5. R ƛ = I P D P P D

Therefore, the optical power is equal to the ratio of the of the diode generated photocurrent and the responsivity of the photosensor, as shown on Equation 6.

Equation 6. P P D = I P D R ƛ

In a default configuration, with the photodiode connected to input I1, and the LOG200 reference current output pin (IREF) connected to input I2, the voltage output is defined by the transfer function shown in Equation 7:

Equation 7. V L O G = 0.250 × l o g 10 I P D I R E F + V R E F

On this LOG200 circuit, IREF of 1µA corresponds to an equivalent effective reference power PREF of 1.25µW for a photodiode having a responsivity of 0.8A/W. Therefore, the LOG200 output transfer function can be expressed as Equation 8:

Equation 8. V L O G = 0.250 × l o g 10 P P D P R E F + V R E F

The VLOG voltage output can be used to calculate the optical power, where the slope for calculation is 250mV/decade for optical power or photodiode current.