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

2.2.3.3 Demodulator and Integrator

Figure 2-9 shows the schematic of the demodulator and integrator stages. The demodulator (multiplier) topology is selected as a chopping switch using two pairs of SPDT analog switches (TS5A623157). The input of the switches are the BPF output and a buffered common mode voltage (1.65 V), respectively. By alternating switches with the LED_DEMOD and LED_DEMODB signals at frequency fmod, the output polarity of the chopping switch relative to the input can be the same or opposite, essentially multiplying the input by +1 and –1, respectively.

The output of the demodulator is fed into an RC integrator. When the reset switch (SN74LVC1G66) is on, the integrator output is reset to VCM,BPF. When the reset switch is turned off (integrator is active), the input voltage of the integrator is converted to current IINT by RINT and charges or discharges the capacitor CINT based on the current polarity. The integrator output is sampled by the ADC before the reset on signal pulse by pulse.

The integrator R and C is designed as 28 kΩ and 12 pF, respectively, to have a target gain value calculated as:

Equation 7. GINT=2Tp,modRINTCINT

Tp, mod is the pulse high duration of the demodulation pulses. The integrator gain is designed to be 16 in this design.

GUID-20231020-SS0I-MWV9-W8HL-NTNTLXJBQ0JG-low.svgFigure 2-9 Demodulator and Integrator Schematic

Figure 2-10 shows the overall transient waveform of the signal chain when smoke sensing is operated. The digital sample Dn,k denotes the sensing signal of the k-th pulse in n-th sensing phase. The final output signal for the single sensing phase can be obtained by passing all the pulses into a digital filter or simply averaging the results. For example, if M is the total number of pulses sent in the given sample phase, the final output is given as:

Equation 8. Dn=∑k=1MDn,kM

Equation 8 indicates that low noise (better performance) can be achieved by sending a higher number of pulses at the cost of power consumption.

GUID-20231020-SS0I-LZ6T-WRJ7-WV0QMTFBJR46-low.svgFigure 2-10 Transient Waveform of Intermediate Signal and Control of the Signal Chain