  This paper presents the trend of biopotential recording front-end channels
developed from the 1970s to the 2020s while describing a basic background on
the front-end channel design. Only the front-end channels that conduct
electrical recording invasively and non-invasively are addressed. The front-end
channels are investigated in terms of technology node, number of channels,
supply voltage, noise efficiency factor, and power efficiency factor. Also,
multi-faceted comparisons are made to figure out the correlation between these
five categories. In each category, the design trend is presented over time, and
related circuit techniques are discussed. While addressing the characteristics
of circuit techniques used to improve the channel performance, what needs to be
improved is also suggested.
