Low Power CMOS Chopper Amplifier for EEG Acquisition Systems
The demand for technologies that help neuroscientists and clinicians to ac- tively observe a large number of neurons in the brain has increased signif- icantly. Currently the biopotential acquisition systems are bulky and not portable that makes it dicult for the on-going diagnostics and causes dis- comfort to the patient. There is a need to make these systems more compact and to extend their applications to various fields. Commonly monitored biopotential signals are EEG, ECG, and EMG. Figure 1 shows the characteristics of these signals, and it can be seen that they have a very low signal amplitude and operate at extremely low fre- quencies (<150 Hz). This is a major reason their performance is limited by the offset and the noise of the input amplifiers. The physical origin of low frequency noise in MOSFETs is the carrier number uctuation theory, or the trapping-detrapping model. It is caused by the uctuation of the number of inversion layer carriers as they are trapped and detraped to and from traps located in the oxide, and it accounts for the carrier number and the mobility uctuations [1]. It should also be noted that the typical o set voltages for these applications is around 10 . As discussed in draft-1, EEG waves are common biopotential signals that are recoreded frequently in modern clinical practise. Due to the low frequency and V level amplitude of these signals, they are often dominated by noise. Also present are the common-mode interference from the mains, as well as electrode-o set. Therefore, designing the analog readout front-end is extremely crucial for these applications. In order to achieve the best signal extraction, a front-end with high CMRR, low-noise, and high-pass filter characteristics is required. The front-end should also have configurable gain and filter considerations.
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