Система для экспериментальной оценки паразитных параметров интегральных мультиплексоров для электроимпедансной томографии

Abstract

Physical parameters of analog multiplexer have influence on measuring, provided for electrical impedance tomography. For bio-medical application the most important is channel stray capacitance and difference among channels. Typical value is around several tens of picofarads. Integrated circuit manufactures usually do not specify this parameter and provide only typical value. Actual stray capacitance can be different from batch to batch up to several tens percentage. Parameter testing and selection can provide better quality of measurement or help to decrease influence of capacitance differences between channels. Leakage currents can cause significant influence on measurements. Especially, multiplexers lightly damaged by static electricity, can increase their leakage currents by an order of magnitude. Software and hardware system for testing of the multiplexers was developed. It can test main parameters of high voltage precision analog multiplexer, like: channels commutation, charge injection during channel switching, channels resistance, capacitance and leakage currents. Channel capacitance measurement accuracy is better than 1 pF, indeed we should substitute stray capacitance of reference multiplexer and empty test-socket. Channel capacitance measurement accuracy is around 1 Ohm, charge injection is around 1 pQ, leakage current around 100 pA. For testing we use reference multiplexer with known parameters, timer to measure charge-discharge time for capacitance estimation, precision amplifier and reed switch to measure small voltage. Replaceable sockets help test analog multiplexer not only 1 to 16 configurations, but any other configuration with smaller number of channels and even lower voltage supply if use voltage stabilizer embedded in the socket. System can check that multiplexer is fine, choose the same parameters of ICs for existing system, improve quality by choosing the ICs with the best parameters or measure them for further compensation by software.