Nowadays, mostly digital amplifiers are used to record biosignals. The amplification process does not only depend on the performance and specifications of the amplifier device, but also closely binds to the types of electrodes to attach on the subject’s body. Types of electrode materials and the mount position of electrodes affect the acquirement of the signals.  Multielectrode arrays are also used, in which multiple electrodes are arranged in an array.
Electrodes made with certain materials tend to perform better by increasing surface area of the electrodes. For instance, Indium tin oxide (ITO) electrodes have less surface area than those made with other materials, like titanium nitride. More surface area results in reducing impedance of electrode, then neurons signals are obtained easier. ITO electrodes tend to be flat with a relatively small surface area, and are often electroplated with platinum to increase surface area and improve signal-to-area ratio.
Digital amplifiers and filters are produced small enough nowadays to be combined with electrodes, serving as preamplifiers. The need for preamplifiers is clear in that the signals that neurons (or any other organs) produce are weak. Therefore, preamplifiers preferably are to be placed near the source of the signals, where the electrodes are adjacent to. Another advantage for having preamplifiers close to the signal source is that the long wires lead to significant interference or noise. Therefore, it is best to have the wires as short as possible. To provide the best protection against the electrostatic noises, shielded cables were used for input, and the shields were connected to the guard drive pins of the INA116 what is this Guard Drive : The guard amplifier drives the inner shield, which serves as the guard shield for the system. … Guard Shielding on Printed Circuit Boards The guard shield concept can be extended to the input pins of high-gain amplifiers on printed … if you wanna know more about read here : 
In practice, electrophysiological recording is frequently impaired by electromagnetic interferences. The most common countermeasure is electrically shielding the recording apparatus and the subjects However, this requires a dedicated recording room equipped with such shields, which thereby restricts the environments where recording is done. Some commercial amplifiers feature a ‘hum filter’ that eliminates the AC power noise. A problem with hum filter is that it also attenuates the corresponding frequency band of the original physiological signal, thereby inappropriately filtering out part of the signal (Figure 4c)
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