The picoammeter consists of two main parts, the picoammeter boards and the Field Programmable Gate Array (FPGA) USB board. These two parts are connected with plastic optical fibers. The FPGA board is designed so that up to 16 picoammmeter boards can be connected to it ICG001 simultaneously, which enables multiple current measurements at the same time. In Fig. 1, two picoammeter boards are shown in full, with a number of other boards designated by dots.
3.1. The picoammeter board
One of the main device requirements we set is very low power consumption during long time measurements, which drove our choice of the picoammeter board components. The current conditioning was done with the LMC6442 (4 μW, Ib=5 fA typ.) operational amplifier , which is ideal for battery powered systems that require a low supply current during long periods of operation. In a similar manner, the core of the board is a MSP430F2013 ultra-low power mixed signal microcontroller with an integrated 16 bit ΣΔΣΔ analog digital converter (ADC) . Low-power signal controllers exist in various types, employing different types of ADCs. The three main types – slope ADCs, Successive Approximation Register (SAR) ADCs and the ΣΔΣΔ ADCs can easily be differentiated by their operational frequencies and the resolutions they offer. Since we wanted high resolution measurements taken at a frequency of about 1 kHz, the natural choice was a ΣΔΣΔ ADC.