EEG8 Multiple biological amplifiers.

EEG8 may also be used for EMG. These 8 channel amplifier units can be stacked up to accommodate as many channels as may be required.  64 or 128 channel EEG systems may easily be configured. Uses are similar to those for the BioAmplifier single channel amplifier. Each EEG8 amplifier channel is similar to a BioAmplifier circuit. One SAM unit is required to serve each two EEG8s.

A 16 channel EEG recording, with close up zoom.

EEG8 Technical detail.
The amplifier for each channel is a complete, individual high quality analogue system - no signal multiplexing is used. Full bandwidth and ultra-low noise amplification are opbtained for each electrode. Low pass, high pass, hum notch filters, gain, un-block, calibration and electrode impedance check facilities are included in each amplifier. All 8 channels in one EEG8 amplifier unit must have the same filter, gain, etc. settings (except where an amplifier is specially configured for eye channel measurement). Noise figures of less than 0.1 uV r.m.s. are achieved by very careful design at the input and optical coupling stages. These figures, coupled with extremely good isolation / common mode rejection result in excellent signal clarity. Nine high pass filter ranges include 0.01Hz for readiness potential measurement, and 30Hz for EMG measurement. 4 low pass anti-aliasing filter settings are provided, which may be factory configured to the specification of the user.

High pass filter.
Each individual amplifier is designed with only one early stage of AC decoupling, the selectable high pass filter. This is made possible by transmitting control lines to the pre-amplifier across the isolation barrier through digital opto-couplers. All subsequent amplification is DC coupled, avoiding unwanted interaction between capacitively coupled stages (which can cause a complicated effect when a transient occurs). The high pass filter is single pole, with 9 selectable ranges.

Low Pass Filter.
This two pole, 12 dB per octave filter is designed to remove high frequencies for anti-aliasing purposes. It is normally configured to the Bessel characteristic, but other modes can be accommodated to individual customer requirements. Four settings may be chosen using the SAM LCD panel. Nominal settings are shown below. The graph demonstrates the 12dB per octave filter characteristics at nominal frequency settings. The fourth position may be a wide band setting, effectively un-filtered, passing frequencies up to 10 KHz.

Using these 12 dB per octave anti-aliasing filters, data acquisition sample rate should be at least 4 times the filter frequency. Thus with the filter set at 40Hz to accommodate EEG into the alpha and beta bands, a sample rate of 160 or 200 per second might be considered suitable. However, if the 'hum' filter is simultaneously employed, the 40Hz filter characteristic is considerably enhanced, so that sample rate of 100 per second could be permissible.

With the 12dB per octave filter set at 200Hz, sample rate of 800 or 1000 per second would be required. Nominal low pass (anti-aliasing) filter frequency selections are shown below, but these may be amended by the customer if other settings are preferable.

 Hum Notch Filter

A twin T active notch filter provides 50 dB per octave reduction at 50 or 60 Hz (as required). The filter is selectable by a switch on the control unit, which sets the hum filter in or out for all channels. In addition to the manual control, electronic operation of this facility is provided via the external computer control socket on the control panel.

 Gain

Maximum amplification provided is normally 200,000, although higher gain is possible. There are nine gain ranges. The x200 position (50mV) is automatically used by the electrode impedance checking facility (which injects an AC signal into the electrode and measures the resulting voltage). Fine gain and offset adjustment is set by a pre-set screw on each amplifier, allowing the output level to be set to suit any A-D converter requirements (nominally +/- 5v).

Internal calibrator.

The active electrode input terminal of all channels is switched to a low impedance 1 Hz 100 uV square wave signal source, while the reference terminal is switched to 'isolated ground' potential. This checks the entire amplifier circuit, and simulates an electrode signal of 100 uV, which may be measured at the A-D converter for calibration purposes. Calibration is selected simultaneously for all channels by a switch on the front of the SAM unit.

Electrode impedance checking.

Operating the impedance check switch causes the active electrode input terminal for each channel to be switched to a 30 Hz 0.5 uA square wave signal source, and reduces the gain to 50 mV F.S.D. The amplified output is passed to a level detection circuit which lights a tri-color LED for each channel on the front of the head amplifier unit, showing green for less than 5k ohms, yellow for less than 10k ohms or orange for greater than 10k ohms. These thresholds may be adjusted by a screw trimmer at the back of each EEG8.The impedance 30Hz output signal for each channel may also be evaluated by software, to more accurately indicate individual electrode impedance (the scale range is 100k ohms). The AC current is fed to consecutive channels in opposite phase, such that the resulting current that flows through the (isolated) ground electrode is minimised. For this reason, if the full compliment of 8 channels for each EEG8 unit is not used, try to balance the same number of both even and odd channel numbers.

Electrode connection

Each 8 channel head amplifier provides 8 active electrode input sockets (white) and 8 associated amplifier reference input sockets (black) using medical grade touchproof connectors. Additionally two linked isolated ground sockets (green) are provided, labelled "Body". A multi-way 26 pin connector carries the 8 + input, - reference and isolated ground lines for each channel allowing direct multi-way connection in large system configurations. A suitable jumper is supplied with each EEG8, allowing all reference inputs to be commoned together when the amplifier is used in common reference mode. With this link in place, any one of the 8 amplifier reference input connectors (black) may be used for connection to the reference electrode/s (e.g. mastoid / ears). Electrode boards and electrode board adaptors have been custom made for easy input connection in large banks of EEG8 (e.g. above 32 channels).

The body connector should be connected to one additional electrode, which can be positioned anywhere on the subject. This connection does not affect the 'reference source' for measurement, its effect is mainly to minimise 'common mode' disturbance to the active amplifier inputs and thus reduce 50/60Hz noise. As a result, EEG8 is capable of producing noise free EEG in extremely adverse circumstances. Owing to the very high input impedance of the amplifier, it will not work properly unless the body connection is present.

EEG8 has 'DC coupled' connection to the electrodes; the body connection is required as a source for 'bias' to the amplifier inputs. The input amplifier has gain of 50 before decoupling, which allows an electrode bias potential of up to 200mV between positive and reference inputs. For this reason, active electrodes and reference electrodes must be of the same type (e.g. all tin, or all silver/silver chloride, or all gold). Body electrode may be any type, but normally would be the same type as others.

Output connection

Analog output for each channel, with an associated ground reference is provided on a 9 way cannon socket. A suitable mating cable is supplied to bring outputs to a remote A-D converter if required. Output voltage may be factory set to any levels required by an A-D converter within the range +/- 10V (normally set to +/- 5V).