A Low-Noise Int-Diff Small Direct Currents Measurement System With Two-Stage Amplifier and Digital Differentiator

Ionization chambers are used as secondary standards in radionuclide metrology. In activity determinations, the measurement of the ionization current is an uncertainty limiting factor, thus requiring precision picoamperemeters with low noise and high stability of transfer coefficient. A small direct current measurement system is presented, including a two-stage amplifier, a filter, a field programmable gate array (FPGA), and host computer software. It is based on the integrating-differentiating (int-diff) scheme, where the integration function is performed through 100 capacitors and an operational amplifier, while the differentiation function is implemented in the FPGA. Test results with a prototype show a low noise level of 7 fA/
$\surd $
Hz at 100 Hz and no significant dc offset in the transient current measurement waveform. The system has a 10 kHz bandwidth for fast settling, and low-frequency currents can also be measured. Results show that the performance of this system meets design expectations, and can apply to ionization current measurements.