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

Abstract

Ultra-low current measurements require careful attention to hardware and materials that are many times inconsequential in a typical circuit and PCB design. The goal of this application note is to discuss the considerations for femtoampere level current accuracy measurements, and provide operational-amplifier based circuits that can be used for calibration of systems that require ultra-low current measurements.

This paper starts from an ideal condition measurement, followed by practical designs that satisfy performance and efficiency for calibration of low current measurements using a common circuit technique called a coulombmeter. Measuring low-level current at this level requires careful analysis and understanding of not only the IB characteristic of the op amp but also the surrounding circuitry to achieve reliable and repeatable results. As a result, it delineates the handling techniques for extremely low-level currents such as tens of atto-Ampere in resolution. The document will push the boundary of low-level current measurement and its application.