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.
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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.