THE ANALOG PROCESSING AND DIGITAL RECORDING OF ELECTROPHYSIOLOGICAL SIGNALS F. Babarada 1 , J. Arhip 2 and C. Ravariu 1 1 University Politechnica of Bucharest, Faculty Electronics Telecommunications and Information Technology, DCAE, ERG, Bucharest, Romania 2 S.C. Seletron Software si Automatizari SRL, Bucharest, Romania Abstract— The paper presents the aspects of design and im- plementation of a chain for electrophysiological signal re- cording, respectively collection, analog processing and digital data recording. The analog processing is based of dynamic range compressor circuit composed by the automatic gain control with a recovery delay to minimize the distortions, thermal behavior compensation and an adaptation stage for levelmeter. At compressor output was added a clipper, which must catch all the transitions, that escapes out of the dynamic range compressor and at clipper output a lowpass filter to cuts abruptly the high frequencies. The data vector recording is performing by strong internal resources microcontroller in- cluding ten bits A/D conversion port. Keywords— bioelectronics, analog processing, digital re- cording. I. THE ELECTROPHYSIOLOGICAL SIGNALS AQUIRING The paper presents an analog processing and digital re- cording system for electrophysiological signals, with the possibility to utilize in medical applications like ECG, EEG, EMG etc, [1]. The electrodes represent the electric conduc- tors together with the contact electrolyte for electrophysio- logical signals collection. For a vector data storage the de- sign of the ensemble source-electrodes-amplifier is very important. So for high contact area electrodes exceeded 100 m square, it is very important to minimize the ampli- fier noise and for low contact electrodes area, the noise introduced by the electrodes begins to be most significant [2]. As the results of modeling of the ensemble source- electrode, it is recommended for the amplifier to be imple- mented by a low noise and distortions, transimpedance amplifier stage followed by one low passing filter. For very low electrophysiological signals it is necessary a differential amplifier because it has a high common mode rejection of parasitic signals characteristic [3]. II. THE ELECTROPHYSIOLOGICAL SIGNALS PROCESSING As in the case of many concepts found in engineering, automatic gain control was also discovered by natural selec- tion. For example, in human vision, calcium dynamics in the retinal photoreceptors adjust gain to suit light levels [4]. A. The automatic gain control Automatic gain control (AGC) is an adaptive system found in many electronic devices. The average output signal level is feedback to adjust the gain to an appropriate level for a range of input signal levels. For example, without AGC the sound emitted from an AM radio receiver would vary to an extreme extent from a weak to a strong signal; the AGC effectively reduces the volume if the signal is strong and raises it when it is weaker. AGC algorithms often use a proportional-integral-differential controller. Other applications include radar, audio/video amplifiers [5] and biological signals processing. The basic components of compressor are the U9, U10 in- tegrated circuits, which biases the D1, D2 diodes, fig. 1, at their I-V curve knee. The input voltage is in the range of 10 to 300mVp and the output voltage is in the range of 5 to 10 mVp. The voltage command of AGC is in the range of 300 to 600mVdc. The resistor R3 allow the circuit to be bal- anced and adjust the output voltage so it does not produce distortion in the output when gain reduction is active. In order to provide the voltage command of AGC (Vcaa) we choose a feedback configuration design. This design contain the amplifier, composed by U11, R4, R5 with the amplifica- tion around 101, the full wave rectifier, composed by D3, D4, R6, R7, U12 which bring the signal to the absolute value and the positive voltage detection realized with D6, C12 connected trough half voltage divider R8, R9. The voltage over the condenser C12 is exactly the volt- age command of the automatic control amplifier Vcaa. Discharging of the condenser C12 is made through the di- ode D7. This diode is opposite polarized by a voltage greater than Vcaa, respectively the voltage produced by diode D5, which is not reduced by half and loaded at the absolute value the condenser C13 through resistances R10 and R11. At reduction of the input signal amplitude the voltage Vcaa remains constant until C13 is discharged by R11 and R10. Thus at transient simulation at 1kHz, the amplification remains constant 5ms and then increase in time of 15ms, fig. 2. P.D. Bamidis and N. Pallikarakis (Eds.): MEDICON 2010, IFMBE Proceedings 29, pp. 132–135, 2010. www.springerlink.com