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

7.2 Sallen-Key Low-Pass Filter

A typical application use case for the auxiliary amplifier is implementing an active low-pass filter and delivering a low-noise signal free of high-frequency aliasing signals into an ADC. Figure 7-2 shows an example of the LOG200 with the auxiliary amplifier on a second-order low-pass Sallen-Key filter. The LOG200EVM provides footprint components to implement this filter in the hardware. This example presents a second-order Sallen-Key filter configuration to provide a Butterworth filter response, giving a 40dB/decade roll-off with a –3dB frequency of 50kHz.

LOG200 with Sallen-Key Low-Pass FilterFigure 7-2 LOG200 with Sallen-Key Low-Pass Filter

Design Notes:

The design is implemented using TI's Filter Design -Tool. The steps to design the filter are as follows:

  1. Use TI's Filter Design -Tool selecting a low-pass second-order filter with Gain 0dB, a passband frequency of 50kHz, and Butterworth filter response.
  2. Using TI's Analog Filter Tool, select the Sallen-Key topology for the filter and create the design.
  3. Scale the resistor values carefully to not meaningfully contribute to the output noise produced by the LOG200. Choose E94 (5%) or better accuracy capacitors and E198 (0.1%) resistors for accurate filter frequency response. Select high-grade C0G (NP0) capacitors on the filter and signal path to minimize distortion.
  4. Set the output resistor to 24.9Ω with a 1.2nF capacitor. The values shown in this example provide good settling performance for the LOG200 and ADS7052 14-Bit, 1-MSPS, single-ended input, SAR data converter. If the circuit is modified, the designer can need to select a different R-C charge reservoir filter depending on the ADC selection and application needs. See the Introduction to SAR ADC Front-end Component Selection training video for more information.

Figure 7-2 shows the filter's gain and phase frequency response plots. The Sallen-Key filter features a Butterworth response with a maximally flat passband gain, and a second-order roll-off at 50kHz. The filter greatly attenuates any unwanted higher-frequency signals and keeps noise from aliasing back into the photosensor signal frequency range.

Aux Amplifier Sallen-Key Filter Frequency Response SimulationFigure 7-3 Aux Amplifier Sallen-Key Filter Frequency Response Simulation