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

4.3.1 Varying Amplifier Speeds

Figure 4-11 shows the SNR at 1 nA with varying time constants and optimized average filter. Using one signal ADC sample, τ1=59 μs and τ2=60 μs has the best SNR at 25.7, a 36% increase over τ1=15 μs and τ2=15 μs at 18.9. This result shows the benefit of optimizing the amplifier time constants. When multiple samples are averaged, the SNR improves further. Averaging 5 samples 20 μs apart starting at 48 μs after the LED is enabled brings the SNR at 1 nA up to 31.1 when τ1=15 μs and τ2=15 μs.

GUID-20200930-CA0I-MMRT-6NWX-XJJR1NCXSRXQ-low.gif

tLED=100 µs tSAMP=20 µs

Figure 4-11 SNR at 1 nA with Varying Amplifier Speeds
GUID-20200930-CA0I-6FZ5-MHLM-NTL0FMTZP9TC-low.gif

tLED=100 µs tSAMP=20 µs

Figure 4-12 Optimal First Top ADC Sample for Each Amplifier Speed