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

2.4.1 ADC Sampling

Taking multiple ADC samples is an effective way to improve the SNR. Comparing configurations 3 and 5 in Table 2-1, the SNR is significantly improved by averaging multiple top ADC samples. The number of top ADC samples is displayed as NTOP and the time of the first top ADC sample is tTOP. Faster ADC sample rates allow more samples to be taken at the top of the photo pulse and improve the SNR further. This is demonstrated in configurations 5 and 7. Table 2-1 displays the time between ADC samples as tSAMP.

Higher ADC resolutions improve the SNR to a limit. Figure 4-21 displays the effect of varying the ADC LSB size for a 260 mV amplitude pulse. The SNR drops when the ADC least significant bit (LSB) is greater than 5 mV. In this case, a 10-bit ADC with a 2 V to 3 V full-scale voltage is sufficient for maximizing SNR.

Taking multiple ADC samples of the signal before the LED is enabled is effective in systems that measure the ambient light level before the LED is enabled. This is displayed as NBASE in Table 2-1. The benefit is shown in Figure 4-20. Because the LED is not enabled for these ADC samples, low power is consumed when taking the multiple ADC samples before the LED is enabled.

If the ADC sample rate is different from the examples in Table 2-1, take samples for the same amount of time as done in Table 2-1. For example, if a 5 µs tSAMP is used with τ1 and τ2 set to 15 µs, take 40 base samples and 16 top samples.