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

2.2 Smoke Concentration Measurement

A smoke concentration measurement follows this general procedure:

  1. Enable the photo amplifier and analog multiplexer (AMUX) buffer.
  2. Take one or more ADC measurements after the photo amplifier and AMUX settles. This measures the ambient light entering the photo chamber. ADC samples taken before the LED is enabled are referred to as base samples in this report.
  3. Enable the LED.
  4. Take one or more ADC measurements after the photo amplifier and AMUX settles. This measures the ambient light and scattered light in the photo chamber. ADC samples taken after the LED is enabled are referred to as top samples in this report.
  5. Disable the LED, photo amplifier, and AMUX buffer.
  6. Process the ADC measurements to determine the smoke concentration. The photo chamber responsivity must be known to obtain an accurate measurement.

The SNR of the smoke concentration measurement depends on many parameters in the system, including amplifier speeds, LED current, LED pulse width, ADC sample rate and digital filtering. These parameters can be adjusted to improve the SNR. The LED and photodiode parameters, such as quantum efficiency and half-angle, and the photo chamber geometry also have an effect on the SNR but are beyond the scope of this report. Table 2-1 lists various measurement configurations and the SNR at 1 nA of photodiode current. The SNR at 1 nA is reported here as a unitless ratio and can be converted to decibels by taking the base-10 logarithm and multiplying by 20. Table 2-1 provides a reference for users to design a smoke detector using the TPS880x AFE. The parameters in the table are discussed in Section 2.3 and Section 2.4. The configurations in Table 2-1 are selected from measurements in Section 4. Section 4 also outlines the measurement process.

Table 2-1 System Configurations and Measured SNR at 1 nA Photodiode Current
Config. tLED (µs) τ1 (µs) τ2 (µs) NBASE NTOP tSAMP (µs) tTOP (µs) Filter SNR at 1 nA
1 50 15 15 5 1 20 68 - 13.0
2 50 15 15 5 2 20 53 Average 15.6
3 100 15 15 10 1 20 91 - 18.9
4 100 59 60 10 1 20 129 - 25.7
5 100 15 15 10 5 20 48 Average 31.1
6 100 15 15 10 7 20 8 Matched 32.3
7 100 15 15 20 8 10 68 Average 33.1
8 200 59 60 20 1 20 216 - 42.5