SNLA399 July   2022 OPA607

 

  1.   Abstract
  2.   Trademarks
  3. 1Introduction to Particulate Matter Sensors
    1. 1.1 Optical Particulate Matter Measurement Instruments
      1. 1.1.1 Photometer
      2. 1.1.2 Optical Particle Counter
        1. 1.1.2.1 Determination of Size and Concentration
          1. 1.1.2.1.1 Size
          2. 1.1.2.1.2 Concentration
    2. 1.2 Architecture of Optical PM Sensor
  4. 2Single Op Amp Based Analog Front End
    1. 2.1 Frequency Response
    2. 2.2 Noise
    3. 2.3 Data Acquisition
      1. 2.3.1 Minimum Current Calculations
      2. 2.3.2 Maximum Current Calculations
  5. 3Optimizing for Different Photodiode Currents
  6. 4Summary

2.3 Data Acquisition

GUID-20220314-SS0I-7HPP-1HBC-WPK4PJ9P2JVP-low.png Figure 2-4 Block Diagram with Charge Bucket

It is assumed that the output voltage of the analog front end is acquired using a single-ended 10-bit ADC available on MSP430G2x53 microcontroller board. The voltage reference of the ADC is 3 V and analog signal sampled at a rate of 10 ksps (100 µs), satisfying the Nyquist criteria fsample ≥ 2 fmax_signal. ADC has an input capacitance of 27 pF, resistance of 1 kΩ, maximum conversion time of 3.5us. The sample and hold circuit can be modeled using RC low-pass filter during the sampling time tsample (100 µs - 3.5 µs = 96.5 µs). It is calculated that a charge bucket of 130 Ω and 570 pF is required to provide enough charge at the input of ADC to match the input voltage. For more details on charge bucket see TI Precision Lab Videos on SAR ADC Input Driver Design.

The results show that the signal chain and charge bucket are able to drive the 10 bit ADC within 0.5 LSB accuracy.

The measurement range supported by the signal chain is dependent upon the ADC bit resolution and reference voltage as discussed in the following topics.