BioAmplifier Technical details. The BioAmplifier is housed in a remote plastic box. The entire amplifier is isolated by a medical grade DC-DC converter and high voltage optical isolation components. It comprises a bipolar input differential instrumentation amplifier coupled directly to the electrode input terminals (medical 1,5mm ‘touchproof’ connectors). This input configuration has been found to provide minimum possible noise, un-measurably high input impedance, and eliminate the blocking and time constant effects of capacitive input coupling. Minute bias current to the input amplifiers is provided via the subject electrodes, thus the isolated ground or ‘body’ terminal must be connected to the subject via a third electrode, or in lower sensitivity applications may be linked directly to the negative (reference) input terminal, to provide necessary current pathway (billionth of milli-amperes) for the input amplifiers. Without anything connected to the amplifier input, the amplifiers may flat line. Electrode potential of up to 200mV between the positive and negative (reference) inputs can be tolerated. If similar electrodes and jell are used for both, difference in electrode potential is well below this level. A 0.5Hz 100uV square wave calibration source may be switched in using a button on the front of the amplifier. This disconnects the subject and applies the 100uV signal directly to the amplifier input, allowing its full function to be checked from source. The first stage of the 8 range high pass filter (HPF) follows directly after the input amplifier (gain 50). The time constant of this stage is controlled by digital information fed across the isolation barrier from the SAM controller (derived from the setting of the HPF control).This capacitive decoupling stage removes DC from the signal before subsequent amplification in the gain control stage. The 8 range gain stage is also controlled by digital lines transmitted across the isolation barrier, allowing a wide range of gain control before the hum and LP filters. Low pass filtering for the prevention of aliasing in digitization, is provided using a 4 pole Bessel characteristic 24dB per octave analog filter, optimized for minimum phase distortion, controlled by digital lines from the front panel control knob. There is also a selectable 60dB T notch filter to reject hum at either 50 or 60Hz. Finally there is a second high pass filter stage, matched to the first using the same digital control lines, which has the effect of completely eliminating any DC (baseline) offset that may have occurred as a result of thermal effects in any of the previous stages. Resulting high pass filter characteristic is 2 pole, 12dB per octave. The un-block control momentarily sets both sections of the high pass filter to the highest frequency setting, 100Hz, thus re-centering the signal in a few tens of milliseconds. Internal trimmers adjust output offset and overall system gain. A feedback linearized analog optical isolator is used to transmit the analog output signal across the isolation barrier to the SAM unit. 15 way high density connectors are used between the BioAmplifier and SAM, using widely available, mass produced connecting cables. +/-5V analog signal is available for output on pin 5 of the SAM 9 way cannon analog socket. |
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