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Systematic Approach for Correction of Design Deficiencies in Particle Detection Electronics
Jacqueline Bulaclac
Mentor: John LaRue & Richard Nelson
The Coulter Counter is used to count small biological particles in solution. The original concept was
based on the increase in the electrical resistance of an orifice as the presence of a biological particle
altered the electrical conduction path. We have worked to adapt this type of device to MEMS
devices. The detection of a particle is proportional to resistance change and the amplitude of the
excitation. The Coulter Counter is currently used for sizing DNA fragments and sorting of biological
cells or cell fragments. While the Coulter Counter traditionally utilizes DC excitation, our research
group’s Summer 2003 research extended this by implementing an amplitude modulation and
demodulation approach for detecting biological particles. The results of the Summer 2003 research
identified a number of design problems that become potentially significant as the channel dimensions
were reduced to the low micron or submicron range. We propose to find solutions to these problems
to support the next generation of devices to be fabricated by this research group. The identified
problems are as follows: (1) Parasitic capacitances in the current source circuit construction and the
effects of the channel’s high resistances, (2) The current demodulation approach utilizes a fourquadrant multiplier and an active four pole low pass Butterworth filter whose input DC level can
exceed the allowable input voltage, and (3) The parasitic capacitances or the frequency-dependent
input impedance of the instrumentation amplifier.