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  Theory of Bioamplifiers, technical details and Types of bioamplifiers

Bioamplifiers:

A Bioamplifier is a device employed to gather and raise the signal integrity of human neurophysiologic electrical activity for output to a variety of sources.

The Bioamplifier is third generation of this broadly versatile, low noise, biological isolation amplifier, appropriate for connection to measuring electrodes joined anywhere on the human body.

Bioamplifier Technical details:

Bioamplifier is housed in a remote plastic box. The whole amplifier is isolated by a medical grade DC-DC converter and high voltage optical isolation mechanism. It includes a bipolar input differential instrumentation amplifier coupled directly to electrode input terminals (medical 1,5mm ‘touch-proof’ connectors). This input configuration has been found to give minimum possible noise, un-measurably high input impedance, and remove the blocking and time constant consequences of capacitive input coupling. Minute bias current to input amplifiers is given through the subject electrodes, therefore the isolated ground or ‘body’ terminal should be joined to the subject through a third electrode, or in lower sensitivity applications might be linked directly to negative input terminal, to give essential current pathway (that is, billionth of milli-amperes) for input amplifiers. Without anything joined to amplifier input, the amplifiers might flat line. Electrode potential of up to 200mV among the positive and negative inputs can be tolerated. When similar electrodes and jell are employed for both, difference in electrode potential is well beneath this level.

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Types of bioamplifiers:

The neurotrophic electrode is a wireless device which transmits its signals transcutaneously. Additionally, it has elaborated longevity of over four years in a human patient, since every component is totally biocompatible. This is limited in the amount of information it can offer, though, since the electronics it employs to transmit its signal (based on differential amplifiers) need so much space on the scalp that merely four can fit on human skull.

Electroencephalography (EEG) includes the placement of numerous surface electrodes on patient's scalp, in an attempt to record the summed activity of tens of thousands to millions of neurons. The EEG has potential for long term utilization as a brain-computer interface, since the electrodes can be kept on scalp indefinitely. The spatial and temporal resolutions and signal to noise ratios of EEG always lagged behind those of comparable intracortical machines; however it has the benefit of not needing surgery.

Electrocorticography (or ECoG) records the cumulative activity of hundreds to thousands of neurons with a sheet of electrodes positioned directly on the surface of brain. In addition to needing surgery and containing low resolution, the ECoG machine is wired, meaning the scalp can’t be totally closed, raising the risk of infection. Though, researchers investigating ECoG claim that the grid "possesses features appropriate for long term implantation".

Some enhancements in the new Bioamplifier comprise:

A) Lowest possible noise attained with bipolar input phase - less than 0.1 micro volts in numerous conditions.
B) Very high input impedance >1000M ohms
C) Excellent refusal of interference improved by remote amplifier positioning (<10 meters between SAM unit and amplifier).
D) 0.01Hz high pass filter contains readiness potentials and so on.
E) Eight high and low pass filter ranges to accommodate broad ranging applications.
F) 4 pole (or 24dB/octave) anti-aliasing filter optimized for the minimum phase distortion.
G) Built-in for electrode impedance checking.
H) Built-in square wave low frequency 100 microvolt electrode terminal calibrator.
I) Control of gain and filter settings by the host computer whenever needed.

Bioamplifier Specification:

•    Noise Level < 1uV RMS unfiltered R.T.I.
•    Input Impedance >1000 M Ohm, 20pF
•    Common Mode Rejection > 100dB
•    Hum Filter 50/60 Hz > 60 dB
•    Voltage Output +/-5V adjustable
•    Subject Isolation > 6000V, 100 M Ohm
•    Calibrator Accuracy 1

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