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

2.2.3.1 TIA

The first stage of the AFE is a transimpedance gain stage consisting of a resistor-feedback TIA as shown in Figure 2-9. The goal of this stage is translating photocurrent to voltage for future stages as well as providing high gain. The op amp used in TIA is TLV9062S, which has a 10-MHz bandwidth, low broadband noise (10 nV/√Hz) and rail-to-rail input and output (RRIO). The low noise and RRIO feature makes sure that the TIA has high dynamic range, and high bandwidth makes sure that there is compatibility with high modulation frequency and excellent stability even with large input photodiode capacitance. TLV9062S also has a shutdown option that keeps power low between smoke-sensing periods. The feedback resistor of the TIA is chosen as 249 kΩ to provide a transimpedance gain close to 249 kΩ. A 1.9-pF feedback capacitor is added to provide the TIA stability with an input photodiode capacitance up to 50 pF. The 3-dB bandwidth of TIA can be calculated as:

Equation 3. f3dB,TIA=12πRTIACTIA=336.4 kHz

The DC bias of the TIA is provided by a resistor divider as:

Equation 4. Vbias,TIA=R1CM,TIAR0CM,TIA+R1CM,TIA×VDD33=98.9 mV

The design choice of DC bias voltage (approximately 100 mV) is mainly determined by three aspects: (1) large DC current (approximately 12.8 μA) caused by ambient light can be tolerated without saturating the TIA, (2) sufficient op-amp bandwidth can still be provided when the input current pulse arrives compared to biasing at 0 V. Large resistor values (10 MΩ + 309 kΩ) limit the total current flow through the biasing branch (0.32 μA), allowing the branch to be always on with low power, (3) keeping the photodiode reverse-bias voltage low helps to reduce the leakage from the photodiode, which is especially important for high-temperature operation. Reverse-bias also reduces the photodiode capacitance detected by the amplifier.

GUID-20231020-SS0I-5NDQ-FQZD-WHWGQNQ6SGHC-low.svgFigure 2-6 TIA Schematic