SBOA597 November   2024 OPA928

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
  5. 2Architecture of Small Current Measurement
    1. 2.1 Coulombmeter
    2. 2.2 Using the Coulombmeter to Determine IB
    3. 2.3 Leakage of Integration Capacitor
  6. 3Benchmarking
    1. 3.1 Point to Point Wiring
    2. 3.2 Shielding
    3. 3.3 PCB Cleaning
    4. 3.4 Temperature Stability
  7. 4Calibration Using a Coulombmeter for Application Circuits
    1. 4.1 Calibration of Common Application Circuits
    2. 4.2 Calibration of Inverting Input
    3. 4.3 Calibration of Non-Inverting Input
    4. 4.4 Determine Resistance of the Capacitor Using Zero-Cross Method
    5. 4.5 Dielectric Absorption and Relaxation
    6. 4.6 Calibration at 85°C
    7. 4.7 Calibration at 25C
  8. 5Summary
  9. 6References

4 Calibration Using a Coulombmeter for Application Circuits

Previous sections described the limits and precautions of the measurement. To realize a full design, the coulombmeter circuit deploys:

  • A polypropylene capacitor for an integrator that resistance was measured as tens of PΩ at the room temperature.
  • Double layer metal shielding to prevent coupling electric field to the high-impedance node.
  • Point-to-point wiring for high-impedance nodes.
  • Cleanliness.

Figure 4-1 shows an example of a PCB that can allow for both standard surface-mount devices, along with point-to-point wiring. The layout specifically around the input high-impedance nodes of the circuit has milled slots with point-to-point wiring; see below.

OPA928 Calibration Board Layout for the High-Impedance Node Figure 4-1 OPA928 Calibration Board Layout for the High-Impedance Node