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

1 Introduction

Optical systems frequently require wide dynamic range current-voltage conversion. Photodiode sensors produce a current output that changes with incident light, where the typical photo current changes orders of magnitude from hundreds of pico-amperes to a few milliamps; therefore, a wide current range spanning several decades of magnitude must be accommodated.

The LOG200 features two logarithmic amplifiers followed by a differential amplifier to convert current signals into a single-ended voltage representing the log-compressed ratio of the two currents. The device incorporates a photodiode bias and dark current correction adaptive biasing circuit, a precision 1µA current reference, precision 1.65V and 2.5V voltage references, and an auxiliary op-amp to facilitate the device's interface with photodiodes. The LOG200 amplifier ratio is 250mV/decade of current-to-voltage conversion. The simplified circuit diagram in Figure 1-1 shows the LOG200 on a typical photodiode current sense measurement application. The block diagram does not show all the decoupling capacitors for brevity.

LOG200 Typical Application Block Diagram Figure 1-1 LOG200 Typical Application Block Diagram
The LOG200 offers a versatile design faster than previous generation logarithmic amplifier designs, offering 90kHz bandwidth at 1nA of input current, ±0.2% max logarithmic conformity error from 10nA to 100µA. With a rise time of 60ns and a fall time of 150ns on a 100nA-1µA step, the LOG200 can quickly detect changes in optical power across many decades of current. For more information on the LOG200 device specifications and features, refer to the LOG200 Precision, High-Speed Logarithmic Amplifier With Integrated Photodiode Bias Data Sheet.