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

4.1 Measurement Procedure

The SNR of eight different configurations using three digital filtering methods is measured to identify optimal SNR techniques and verify the trends in the calculations. For each measurement, the RPH gain resistors are 1.5 MΩ, gain stage amplification is 32.25, AMUX buffer capacitor is 1 nF, and the CPH compensation capacitor and AMUX buffer resistor are varied to achieve the τ1 and τ2 in Table 4-1. A photoelectric smoke sensor from a commonly available smoke alarm is attached to a TPS8802EVM and sealed to prevent dust from affecting the signal level. The photodiode signal is generated from internal reflections in the chamber. The photodiode current is estimated to be 2.7 nA. All tests use a 3.0 V VBAT supply and 100 mA LED current, and the photodiode capacitance is measured to be 51 pF.

The noise is measured under two system conditions. The ideal system is powered with a 3.0 V supply connected to VBAT, has the photo amplifier continuously enabled and a signal generator connected to LEDEN to enable the LED. The real system uses a MSP430F5529 Launchpad to enable the photo amplifier, LED, and boost converter to replicate the conditions of a real smoke alarm with a 2.0 V supply voltage on VBAT. The boost converter is briefly enabled before enabling the LED to charge the LED supply and VCC capacitors.

The buffered AMUX voltage is probed with an oscilloscope to capture 100 kilosamples at 100 megasamples per second. 100 waveforms of each condition are captured. The samples are centered on the LED_EN rising edge, and the LED_EN pulse is extended by 15 μs to accommodate for the TPS880x LED driver propagation delay. This captures 500 μs of the signal before the LED is enabled and 500 μs after the LED is enabled. The samples are imported into MATLAB, where they are processed to simulate a microcontroller’s ADC.

Table 4-1 Eight Measurement Configurations
# tLED (μs) τ1 (μs) τ2 (μs) System
1 50 15 15 Ideal
2 100 15 15 Ideal
3 100 33 15 Ideal
4 100 33 30 Ideal
5 100 33 30 Real
6 100 59 30 Ideal
7 100 59 60 Ideal
8 200 59 60 Ideal
GUID-20200930-CA0I-K9NR-3S4M-MXKDPGZXNJNF-low.pngFigure 4-1 LEDEN and CSA Voltage (LED Current) for a 100 µs LED Pulse
GUID-20200929-CA0I-QPMV-LQ2W-JLPWVF5NQCDR-low.gifFigure 4-3 20 of 100 Waveform Captures of Configuration 4
GUID-20200930-CA0I-LCZ3-8G34-MJTBQQZJHCDH-low.gifFigure 4-5 Measured Noise Level of Each Time Constant
GUID-20200930-CA0I-CCBM-R2QF-BWGD3KDDZ5GX-low.pngFigure 4-2 Buffered AMUX Pulse With Real Alarm Conditions
GUID-20200930-CA0I-MXJR-FKCR-2FTPQZM6LSVT-low.gifFigure 4-4 Single Waveform Captures of Each Time Constant