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

2.1 Frequency Response

The filter response of this circuit is a bandpass filter composed of simple poles and zeroes. The passband frequency range of the signal chain is from 0.1 Hz to 1 kHz as shown in Figure 2-2. The upper cutoff frequency is important to restrict the noise bandwidth of the circuit. The lower cutoff frequency is required to remove DC offset.

Figure 2-2 Frequency Response of AFE

The feedback capacitance (Cf = 1.5 pF) in parallel to the feedback resistor (Rf = 60 MΩ) is required to avoid any instability issues in the TIA. For details on stabilizing a TIA using feedback capacitance see What You Need To Know About Transimpedance Amplifiers – Part 1. The feedback network generates a pole given by,

Equation 2. p1=12πRfCf=12π x 60 x 106 x 1.5 x 10-12=1.7 kHz

The RC low pass filter placed at the output of operational amplifier increases the fall off rate to 40 dB/dec helping in reducing the output voltage noise thereby, improving the SNR. The RC low pass filter (roll-off filter) gives a pole at frequency given by,

Equation 3. p2=12πR1C1=12π x 1 x 103 x 100 x 10-9=1.6 kHz

The two poles around 1.6 kHz results in a 3 dB point around 1 kHz. Therefore, the upper cutoff frequency of the AFE settles to 1 kHz.

The high pass filter at the operational amplifier output end removes the DC offset due to diode dark current, input bias current and offset voltages of the operational amplifier. Thus only the photo current pulses due to the presence of particle are digitized using ADC. This helps in making the calibration algorithm independent of DC offsets.

A high pass RC filter, inserts a zero at origin and a pole at high pass cutoff frequency which can be adjusted using,

Equation 4. fh=12πR2C2=12π x 100 x 103 x 15.9 x 10-6=0.1 Hz

The particle flow rate range requirement of the PM sensor relates to passband frequency range of AFE. Maximum flow rate of particles sets limit for upper cutoff frequency and minimum flow rate sets the lower cutoff frequency,