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

4.3.3 Varying LED Pulse Length

Varying the LED pulse length tLED has a significant effect on the SNR. When scaling tLED, τ1 and τ2 by two, the SNR is expected to improve by the square root of two. With one ADC sample, the SNR/nA increases from 13.0 to 23.3 to 42.5 when tLED is increased from 50 μs to 100 μs to 200 μs. However, this also increases the power consumption, since the LED is enabled for more time. If the LED current is scaled to keep the power consumption constant, assuming that the photodiode current increases proportionally to the LED current and the LED supply voltage is constant, the 50 µs pulse has the highest SNR as shown in Figure 4-16. In general, increasing the LED pulse length and amplifier time constants is an effective way to increase the SNR when power consumption is flexible.

GUID-20200930-CA0I-S74B-PCSG-FPZNJQ8FBB0P-low.gif

tSAMP=20 µs

Figure 4-15 SNR at 1 nA Using Optimal Averaging Filter Varying LED Pulse Width
GUID-20200930-CA0I-SMQ3-B1Z5-QWB1GQLKHH6H-low.gif

tSAMP=20 µs

Figure 4-16 SNR at 1nA Using Optimal Averaging Filter Varying LED Pulse Width and LED Current