  A 128-channel picoammeter system is constructed based on instrumentation
amplifiers. Taking the advantages of high electric potential and narrow
bandwidth in DC energetic charged beam measurements, current resolution better
than 5 fA can be achieved. Two 128-channel Faraday cup arrays are built, and
are employed for ion and electron beam current distribution measurements. Tests
with 60 keV O3+ ions and 2 keV electrons show that it can provide exact
boundaries when a positive charged particle beam current distribution is
measured.
