SLVAEX3 October   2020 TPS8802 , TPS8804

 

  1.   Trademarks
  2. 1Introduction
  3. 2SNR Optimization
    1. 2.1 SNR Overview
    2. 2.2 Smoke Concentration Measurement
    3. 2.3 Amplifier and LED Settings
      1. 2.3.1 Photo Amplifier Gain
      2. 2.3.2 Photo Amplifier and AMUX Speed
      3. 2.3.3 LED Current and Pulse Width
    4. 2.4 ADC Sampling and Digital Filtering
      1. 2.4.1 ADC Sampling
      2. 2.4.2 Digital Filtering
  4. 3System Modeling
    1. 3.1 Impulse Response
      1. 3.1.1 Photodiode Input Amplifier Model
      2. 3.1.2 Photodiode Gain Amplifier and AMUX Buffer Model
      3. 3.1.3 Combined Signal Chain
    2. 3.2 Noise Modeling
      1. 3.2.1 Noise Sources
      2. 3.2.2 Output Voltage Noise Model
      3. 3.2.3 ADC Quantization Noise
    3. 3.3 SNR Calculation
      1. 3.3.1 Single ADC Sample
      2. 3.3.2 Two ADC Samples
      3. 3.3.3 Multiple Base ADC Samples
      4. 3.3.4 Multiple Top ADC Samples
      5. 3.3.5 Multiple ADC Sample Simulation
  5. 4SNR Measurements
    1. 4.1 Measurement Procedure
    2. 4.2 Measurement Processing
    3. 4.3 Measurement Results
      1. 4.3.1 Varying Amplifier Speeds
      2. 4.3.2 Varying Digital Filter and ADC Timing
      3. 4.3.3 Varying LED Pulse Length
      4. 4.3.4 Varying ADC Sample Rate
      5. 4.3.5 Real and Ideal System Conditions
      6. 4.3.6 Number of Base Samples
      7. 4.3.7 ADC Resolution
  6. 5Summary
  7. 6References

1 Introduction

In a photoelectric smoke sensor, an LED periodically emits light into a chamber. The light scatters off smoke particles and enters a photodiode. The photodiode outputs a current proportional to the incident light, which is proportional to the smoke concentration. The LED current must be pulsed and regulated, and the photodiode current must be amplified with low noise and high linearity. Therefore, smoke detectors require a LED driver and photodiode amplifier to obtain an accurate signal from the sensor. With the signal amplified, an analog-to-digital converter (ADC) on the MCU captures samples of the signals and the MCU calculates the smoke concentration.

The TPS880x AFE integrates the regulators, drivers and amplifiers to interface the smoke detector’s power supply, sensors, and microcontroller unit (MCU). The TPS880x has adjustable LED driver current, LED pulse width, amplifier bandwidth, amplifier gain, and output filter bandwidth. Each of these parameters, along with the ADC timing, affects the SNR. When all of these parameters are adjusted, reliable smoke sensing with only nanoamps of photodiode current is achieved.

This report analyzes how each of the system parameters affects the SNR. Modeling the system provides valuable insight on how to optimize the SNR. Measurements verify the trends in the model and provide configurations for users to start their designs.
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