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

2.3.2 Maximum Current Calculations

The absolute maximum reading supported by the signal chain is given by the value of current which saturates the output of amplifier to supply voltage. So absolute maximum current (Iabs) is given by,

Equation 7. Iabs=VCCRf= 3.360 x 106=55 nA

The AC current reading supported by TIA in such case is,

Equation 8. Iin_max= VCCRf- Ioffset

where Ioffset refers to the DC offset contributed by the diode dark current, offset voltage, input bias current and any other illumination current even in the absence of particle depending upon the optical chamber construction. For this simulation diode BWP34S was assumed, and it was considered that even with the exponential increase in current with rising temperature, the offset current (Ioffset ) would be below 1 nA.

Maximum AC current limit is related to ADC by the expression,

Equation 9. Iin_max ≤ VrefRf= 360 x 106=50 nA

as Vref is the maximum input voltage which can be converted by the ADC.