A biopotential is a voltage produced by a tissue of the body, particularly muscle tissue during a contraction. Electrocardiography depends on measurement of changing potentials in contracting heart muscle. Electromyography and electroencephalography function similarly in the diagnosis of neuromuscular and brain disorders, respectively.
Biopotentials can be read by a bioamplifier, an electrophysiological device, a variation of the instrumentation amplifier, used to gather and increase the signal integrity of physiologic electrical activity for output to various sources. It may be an independent unit, or integrated into electrodes.
Electroencephalography (EEG) instrumentation acquires signals from muscles below the skin generated by brain cells. Simultaneously, EEG records the summed activity of tens of thousands to millions of neurons. As the amplifiers became small enough to integrate with the electrodes, EEG has become to have the potential for long term use as a brain-computer interface,
High performance differential amplifiers are used for amplification. Signals of interest are in the range of 0.5–100 V, over the frequency range of 1–50 Hz. Similar to EMG amplifiers, EEG benefits from the usage of integrated circuit. The chances of EEG is also mainly from the asymmetrical placement of electrodes, which leads to increased noise or offset. Some minimal specifications for a modern EEG amplifier includes:
Low internal voltage and current noise(<1 mV, 100 pA), High input impedance (>108 MΩ), Bandwidth (1–50 Hz), Frequency cutoffs (>18 dB/octave), High common mode rejection ratio (>107), Common mode input range (greater than ±200 mV), Static electricity shock protection (>2000 V), Gain stability > ±1%
Galvanic skin response
Galvanic skin response is a measurement of the electrical conductance of the skin, which is directly influenced by how much the skin is moist. Since the sweat glands are controlled by the sympathetic nervous system, the skin conductance is crucial in measuring the psychological or physiological arousal. The arousal and the eccrine sweat gland activity are clinically found to have direct relation. High skin conductance due to sweating can be used to predict that the subject is in a highly aroused state, either psychologically or physiologically, or both.
Galvanic skin response can be measured either as resistance, called skin resistance activity (SRA) or skin conductance activity (SCA), which is a reciprocal of SRA. Both SRA and SCA include two types of responses: the average level and the short-term phasic response. Most modern instruments measure conductance, although they both can be displayed with the conversion made in circuitry or software.
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.
However, when wider bands are needed, for instance a very high (action potentials) or a low frequency (local field potentials), they could be filtered digitally, perhaps with second-stage analog amplifier before being digitized. There may be some drawbacks when several amplifiers in cascade. It depends on the type, analog or digital. However in general, filters cause time-delay and amendments are needed to have signals in sync. Also, as extra complexity is added, it costs more money. In terms of digital amplifiers, a lot of works that the laboratories do are feeding back signals to the networks in closed loop, real-time. As a result, more time is needed to apply on signals when there are more digital amplifiers on the way. One solution is using field-programmable gate array (FPGA), the “blank slate” integrated circuit that is written whatever on it. Using FPGA sometimes reduces a need to use computers, resulting in a speed-up of filtering. Another problem with cascaded filters occurs when the maximum output of the first filter is smaller than the raw signals, and the second filter has a higher maximum output that the first filter. In that case, it is impossible to recognize if the signals have reached the maximum output or not.