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

4.2 Calibration of Inverting Input

The first case occurs when the high impedance node is connected to the inverting input terminal. These typical cases are leakage current measurements (or insulation resistance), a trans-impedance amplifier, and piezoelectric element amplifier.

Application Circuits for High-impedance Inverting Input Terminal Figure 4-2 Application Circuits for High-impedance Inverting Input Terminal

As shown in the drawing, disconnect the sensor and transform the circuit into a feedback loop with a buffer amplifier with gain. Short the non-inverting input pin to the ground.

Configuration for Inverting Input Terminal Calibration Figure 4-3 Configuration for Inverting Input Terminal Calibration

Insert an integration capacitor between the inverting input and Vcal. So that the circuit is in the calibration mode.

Inverting Input Terminal Calibration Using a Coulombmeter Figure 4-4 Inverting Input Terminal Calibration Using a Coulombmeter