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

1 Introduction

Many systems require ultra-low current amplifiers connected directly to a sensor or probe to achieve the maximum possible measurement accuracy. For this case, precision operational amplifiers can offer the best possible design to buffer a front-end probe or measure ultra-low currents. Precision Operational Amplifiers are designed to perform as closely to an designed for amplifier as possible. One key element of designed for amplifiers is infinite input impedance. For standard CMOS op amps, IB ranges from several pico-amperes (pA) to hundreds of pA. These current levels can be achieved without advanced design considerations. However, additional circuitry and design details are required to achieve the lowest IB performance. For high performance devices, such as the new OPA928, the IB can be less than 1fA, into atto-Ampere levels. The input impedance is high enough to support a variety of high-impedance sensors.