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

4.3.5 Real and Ideal System Conditions

SNR measurements taken using laboratory conditions is expected to provide a better SNR than in a real smoke alarm. For this reason, a set of measurements are taken using a 2V power supply, the TPS8802 boost converter, and a MSP430F5529 Launchpad microcontroller to control the TPS8802. The buffered AMUX output is measured using an oscilloscope. Figure 4-18 displays the 100 waveform average of each condition, showing that the pulse shape is nearly identical. The achieved SNR is higher using the real system, shown in Figure 4-19. No SNR disadvantage is observed from enabling and disabling the photo amplifier, AMUX, and boost converter as done in a smoke alarm.

GUID-20200930-CA0I-KZMD-DF98-4C35SXS95N3G-low.gif

tLED=100 µs τ1=30 µs τ2=30 µs

Figure 4-18 Comparison of Noiseless Real and Ideal System Condition Pulse Shape
GUID-20200930-CA0I-L0DK-H1CR-VWRKJ8D8FVNX-low.gif

tLED=100 µs τ1=30 µs τ2=30 µs

Figure 4-19 SNR at 1 nA for Real and Ideal System Conditions
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