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

3.2.1 Noise Sources

The noise generated by the photo input amplifier and gain resistors RPH generally exceed the noise generated by the photo gain stage, AMUX buffer, and photodiode. The photo amplifier schematics with noise sources modeled is displayed in Figure 3-4. The photo input amplifier is modeled with input voltage and current noises. Op-amp voltage noise is displayed as VNAMP and current noises are displayed as INAMP. The two amplifier current noises are assumed equal, as the op-amp has voltage feedback.

Thermal noise is a noise source that naturally occurs in resistors. Thermal noise is modeled as a voltage noise in series with the resistor or equivalently a current noise in parallel with the resistor. The thermal noise sources in the photo input amplifier stage are shown in Figure 3-4 as INRPH. Here, a current noise is used because the current-to-voltage amplifier transfer function is calculated in Equation 5. The thermal noise at 300 K is calculated in Equation 12.

GUID-20200930-CA0I-ZCRJ-MRVX-RJL9NZRB9P1R-low.gif Figure 3-4 Photo Input Amplifier With Noise Sources Modeled
Equation 12. I N R P H = 0.13 R P H n A H z