Compressive Sensing system for recording of ECoG signals in-vivo

Appendix EE.1. AmplificationThe small amplitude of neural signals and the high impedance of the electrode-tissue interfaceforce to use a front-end amplifier before processing of digitizing the input signal [33]. The mainfeatures are required in the LNA to be used are shorted in Table E.1.1.FeatureNeural Signal AmplificationInput referred noiseLow to revolve spikes of 30 μVDynamic range± 1-10 mV to convey EEG peaksInput impedanceHigher than the electrode-tissue interfaceDC input currentNegligibleFrequency band Depending on signals of interest (see Table 2.1.1)Power ConsumptionLow to avoid neural tissue heatingCommon Mode Rejection High in order to minimize interference from power lineRatio (CMRR)noise and close proximity between electrode andPower Supply Rejection amplifier to minimize capacitive and inductive coupledRatio (PSRR)interferometersTable E.1.1. Main features of a front-end amplifier for neural recording.Besides the characteristics listed behind, the LNA has to block DC offset present at theelectrode-tissue interface to prevent saturation of the amplifier and it has to consume littlesilicon area and use few or no off-chip components to minimize the dimensions. An example ofOTA-based neural amplifier was presented in 2003 by Harrison and Charles.E.2. Signal Digital ConversionIn order to achieve a robust transmission of the signals which have been registered, theamplified and eventually compressed information has to be digitized. There is an extensivevariety of techniques for performing analog-to-digital conversion which are included in Fig.E.2.1.Choice of one specific technique depends on the signal of interest, as well as the area andpower constraints which have to be faced by the designer. As it is depicted below, a data rate of15 kS/s is enough in most of clinical applications, but it requires a data stream of 15Mb/s for 100electrodes, which is impossible to transmit across the skull and the scalp. RF links are limited bythe tissue electromagnetic absorption, which follows an f 2 ratio.84