Ultra-Low Current Sensing from Femtoampere to Picoampere Range

Abstract

This work proposes design steps to form a sample analog electrometer front-end for an ultra-low current sensor in a 0.18μm CMOS technology, which is able to measure from femtoampere to picoampere range. The performance of ultra-low current sensor interfaces can be significantly deteriorated by leakage and noise of the system. The presented design offers significantly lower leakage current, which is independent of the input signal and variations in design parameters such as gain and offset, compared to the conventional integrator. In addition, the input current noise of the system is reduced by controlling the closed-loop bandwidth of the proposed interface. Analysis and simulations show the correct operation for the current values from 1fA to 1pA range with 11 bits linearity, the sensitivity of 400aA, and the input-referred maximum current noise of 360aA.