The further development of simulator programs for electronic and electric circuits gives opportunities for very close to real laboratory experimental work. New problems are emerged with models of parts that previously were not being object of simulation. The paper is provoked from problem in simulation of voltage measurement system that measure voltages in 4 ranges in ISIS Proteus software package. The first attempt of modelling the measuring system gave very disappointing simulation results: the error from measurement was more than 4 percent. The necessity to investigate the sources of this error to reduce it in acceptable limits is aroused. The paper gives recommendations for designers, researchers, scientists and every user of similar simulator software to take into account parameters of such simple elements of the circuits as switches. The choice of the Op Amp in input amplifier is also very important when a minimum error in simulation of measurement system is needed. The focus on investigations is analogue part of the electronic system but the whole circuit with A/D Converter, microprocessor part and indication is described. The aim is to show advantages that ISIS Proteus simulation package suggest for modelling of complex electronic devices.