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

3 Optimizing for Different Photodiode Currents

The signal chain can be modified for the desired current range and transimpedance gain, using the equations explained in Section 2. To modify the signal chain to measure values lower than 50 pA, Rf should be increased. The increment in Rf would result in a decrease the maximum measurable current supported by the signal chain with the same ADC. Table 3-1 mentions different measurement ranges supported by the signal chain with different feedback resistance for a 10-bit ADC at 3-V reference. The capacitance values in the Table 3-1 are for maintaining an upper cutoff frequency equal to 1 kHz.

Table 3-1 Suggested Rf-Cf Combinations for Different IIn
Feedback Resistor (MΩ) Feedback Capacitor

(pF)

 Minimum Measurable Current (Iin_min in pA) Maximum Measurable Current (Iin_max in nA) Input Refereed Current Noise (pA)
300 0.3 10 10 1.8
60 1.5 50 50 2.14
30 3 100 100 2.5

Using a higher Rf with the same ADC enables to achieve a better resolution for detecting minimum change in the input current. For instance, if required range is 50 pA to 5 nA, measuring using Rf = 300 MΩ ensures that every 10 pA change is detectable. Also for the same current value, a larger Rf results in better SNR because increasing Rf reduces the input referred current noise. The graph in Figure 3-1 shows the increasing trend of SNR for 50 pA input signal with increasing feedback resistance.

Figure 3-1 SNR vs. Feedback Resistance