The biological amplifier

Abstract

Biopotentials are essential ssiignals that are immersed in the body like underwater currents in an ionic ocean. Electrodes allow their detection by transducing these ionic phenomena into electrical signg nals, feasible to be processed by electronic circcuuits. Thus, we do not have direct accessss to biopotentials but through variable (and ssometimes unpredictable) electrode skin impedances. Because of this, when available aass electrical signals, biopotentials become faint and vulnerable to noise and interferencce e sources. One approach to bypass this problem is to use invasive techniques such as skin abrading or percutaneous needles inserrt tion, which bring us closer to the electrical soource, thus reducing the impedance. Theesse solutions try to find out a solution comproommising the patients’ comfort, instead of wworking on electronic design. It is a task for bioengineers to design bioamplifiers ablee to acquire high quality signals using non-invasive techniques while dealing with high electrode impedances. This chapter de escribes the characteristics of biomedical ssiignals available at the electrodes, the influence oof external interference sources, and the d desired bioamplifiers’ features to deal with. CClassic and current amplifier design and new tendencies are here in presented, showing circuits for both traditional wet electroddees and capacitive electrodes. The latter is an emerging technology that does not requuiire electrolytes (gel, liquid or paste), which alloow picking up biopotentials without any skin preparation, even through dielectric films or cotton clothes. A note should be mmade about the term “bioamplifier” itself. Whhen analog electronics were the predommiinantly available tool, biopotential measuremments were improved by enhancing the aamplifier design. This remains true, but in a time when digital electronics and integrated systems are ever cheaper and more efficient, we think about the bioamplifier in connection with a broader class of devices which encompass the digital domain. In this chapter we will discuss biopotential acquisition systems, because the characteristics of data converters and mixed signal devices have become an integral part of a “bioamplifier” design