The Amplifier Input Protection Circuit for a Intraoperative Evoked Potential Monitoring System I. Krois * , V. Išgum ** , M. Cifrek * * University of Zagreb, Faculty of Electrical Engineering and Computing, Zagreb, Croatia **University Hospital Rebro/Department of Neurology, Zagreb, Croatia e-mail: igor.krois@zemris.fer.hr , visgum@rebro.mef.hr , mario.cifrek@zesoi.fer.hr Abstract—In this paper a short introduction into problem of intraoperative interference will be presented. Three most important types of interferences will be analyzed. Connection between their physical properties and scheme of protection circuit will be shown. At the example of realized protection circuit we will show what are additional problems in hardware realization. I. INTRODUCTION Our basic intention was to make considerably quality improvement of recording of evoked potential in intraoperative environment. One of the main distinctive parameters between an ambulance and intraoperative system for evoked potentials monitoring is in the level of amplifier input protection. Input protection for intraoperative measurement ought to be much tougher regarding much higher energy levels of interference. Quality of amplifier protection circuit basically can be described by the value of input impedance and noise. Impedance should be as large as possible while for noise opposite should stand. Unfortunately energy absorbing capability of protective components is proportionally related with their capacitance. Therefore their larger energy absorbing capability means smaller input impedance. For the value of noise there is not such straightforward path of reasoning. Noise depends not only on type of components and their typical values but also on scheme of protection circuit. So there are lot of combinations and we have should tried some of them to find optimum with the lowest noise. After that we checked this optimal solution for interference energy level and input impedance. It is clear that the process of amplifier protection circuit design was iterative. II. TYPES OF INTERFERENCES In operating room there are lot of instruments and equipments and all of them produce some level of interference. From the point of circuit protection the most important interferences are those produced by equipments directly attached on a patient body. Direct contact assures that a lot of energy can be easily transmitted between source of interference and patient body. We can make list of most common types of interferences ranked by energy content. A. Defibrillation Defibrillator by far poses the highest treat to amplifier electronic circuits. Its output gives high voltage long lasting pulses (5 kV, >200 ms) which means a lot of energy entering into amplifier inputs. B. Transcranial electric stimulation Transcranial electric stimulator is very similar to defibrillator. Both of them share similar output stage basically made of high voltage capacitor and output switch. Difference lays in the energy level of transcranial pulses. There are about order of magnitude lower than the defibrillation ones. C. Electrosurgical device output signal Electrosurgical device pose a different problem for amplifier protection circuit. Its output is not single pulse but relatively long (usually up to 15 seconds) interval of high frequency signal. How long will interval last depend on a surgeon decision. Output signal is of sinusoidal shape frequency between 500 kHz and 2 MHz with amplitude usually not more than 500 V. There are two basic mode of operation. In burst mode output signal is made of high repetition short lasting bursts. In continuous mode output is one uninterrupted signal interval. III. PROTECTION PRINCIPES Clamping and crowbar are two basic principles of protection from over voltage. In standard circuits for amplifier protection clamping is commonly used. Such a circuit is shown on Fig. 1. + - com Diff. amp. U out R s U + U - R s Figure 1. Standard circuits for protection of amplifier inputs. 0-7803-8271-4/04/$20.00 ©2004 IEEE 75 IEEE MELECON 2004, May 12-15, 2004, Dubrovnik, Croatia