Ansgar T. Kirk

Low-noise current meters are usually designed using high value feedback resistors. However, a high resistance reduces the maximum measurable current at a given output voltage and the maximum bandwidth at a given parasitic capacitance. Capacitive transimpedance amplifiers integrating the current to be measured correspond to a nearly infinite resistance. Here, we present a novel active reset architecture for discharging the integration capacitor that eliminates the leakage currents and charge injection of the necessary switches. This enables a capacitive current meter achieving the noise current of an ideal resistive transimpedance amplifier with an ideal resistance of 650 GΩ, while the dynamic range, bandwidth and zero-point stability are orders of magnitude greater due to the capacitive operating principle. At a 3-dB bandwidth of 50 Hz, the noise current standard deviation is σ i = 2.6 fA {\sigma _{i}}=2.6\hspace{0.1667em}\text{fA} with a dynamic range of six orders of magnitude from femto- to nanoamperes. Digitally adjusting the 3-dB bandwidth to 0.45 mHz for measuring direct currents reduces the noise by three orders of magnitude down to σ i = 8.7 aA {\sigma _{i}}=8.7\hspace{0.1667em}\text{aA} , resulting in a dynamic range of nine orders of magnitude. This is possible due to an excellent zero-point stability within ± 25 aA without temperature or humidity compensation.