TIDUF52 December   2023 MSPM0L1303 , MSPM0L1304 , MSPM0L1305 , MSPM0L1306 , MSPM0L1343 , MSPM0L1344 , MSPM0L1345 , MSPM0L1346

 

  1.   1
  2.   Description
  3.   Resources
  4.   Features
  5.   Applications
  6.   6
  7. 1System Description
    1. 1.1 Key System Specifications
  8. 2System Overview
    1. 2.1 Block Diagram
    2. 2.2 Design Considerations
      1. 2.2.1 Photoelectric Smoke Detector Background – DC-Based Signal Chain
      2. 2.2.2 Modulation-Based Smoke Detection Signal Chain
      3. 2.2.3 Optical Sensing AFE Design
        1. 2.2.3.1 TIA
        2. 2.2.3.2 BPF
        3. 2.2.3.3 Demodulator and Integrator
        4. 2.2.3.4 LED Driver
      4. 2.2.4 Optical and Mechanical Design
    3. 2.3 Highlighted Products
      1. 2.3.1 MSPM0L1306
      2. 2.3.2 TLV9062S
      3. 2.3.3 TPS7A24
      4. 2.3.4 TS5A623157
      5. 2.3.5 SN74LVC1G66
      6. 2.3.6 HDC2010
  9. 3Hardware, Software, Testing Requirements, and Test Results
    1. 3.1 Hardware Requirements
      1. 3.1.1 Power
      2. 3.1.2 Communication Interface
      3. 3.1.3 Headers
    2. 3.2 Software Requirements
      1. 3.2.1 Getting Started Firmware
      2. 3.2.2 Measurements and Smoke Detection
      3. 3.2.3 Additional Demonstration Functionality
      4. 3.2.4 Smoke Detector GUI
    3. 3.3 Test Setup
      1. 3.3.1 UL217 Smoke Box and Fire Testing Setup
      2. 3.3.2 Ambient Light Testing Setup
      3. 3.3.3 Air-Quality Sensing Test Setup
    4. 3.4 Test Results
      1. 3.4.1 UL217 Testing Results
      2. 3.4.2 Ambient Light Testing Results
      3. 3.4.3 Air-Quality Sensing Test Results
      4. 3.4.4 Power Testing Results
      5. 3.4.5 Fire Room Smoke Testing
  10. 4Design and Documentation Support
    1. 4.1 Design Files
      1. 4.1.1 Schematics
      2. 4.1.2 BOM
      3. 4.1.3 CAD Files
    2. 4.2 Tools and Software
    3. 4.3 Documentation Support
    4. 4.4 Support Resources
    5. 4.5 Trademarks
  11. 5About the Author

2.2.3 Optical Sensing AFE Design

Optical power transfer ratio (PTR) is often adopted to represent the smoke intensity. PTR is given as:

Equation 1. PTR=IPD/SλILED×ηLED

where

  • IPD is the photodiode current
  • ILED is the LED current
  • Sλ is the photodiode responsivity at given wavelength
  • ηLED is the LED efficiency

Typically, the smoke alarm trigger level ranges from 0.5 to 5 nW/mW across various fire types. This translates to a corresponding photocurrent of 5 to 100 nA if 100 mA is used for the LED drive current. The target noise performance in this design is less than 50 pARMS so that a 20-dB SNR can still be achieved even at the minimum current scenario for robust smoke alarm triggering. Therefore, a total signal chain gain of 5.8 MΩ is targeted in this design to achieve the target noise performance by minimizing the impact of ADC quantization noise.

Figure 2-5 shows the block diagram details that are adopted in the smoke detector signal chain. The overall signal chain gain can be calculated using Equation 2.

Equation 2. VoutIsmoke=4π2GTIAGBPFGINT

where

  • Ismoke is the photocurrent induced by scattered light from smoke particles
  • Vout is the integrator output voltage before ADC samples
  • GTIA is the Transimpedance Amplifier (TIA) gain
  • GBPF is the bandpass filter (BPF) gain
  • GINT is the integrator gain
GUID-20231020-SS0I-WGQR-RP39-3FQHHVLBJKVW-low.svgFigure 2-5 Modulation-Based Smoke Detector Signal Chain Block Diagram