eego amplifier EE-21x EE-22x User Manual
Rev. 5, DRN-PDO-1664 13 of 26 2016-07-11
2.2 BIOSIGNAL AND IMPEDANCE MEASUREMENTS
Measurements of physiological parameters such as EEG require highest quality and fidelity of the
electronic equipment and sensors used with it. The eego amplifier has been developed on the basis
of latest state of the art core electronic components (integrated circuits). The modern chip-set
allowed miniaturizing hardware without compromises in signal quality and keeping the power
consumption at a minimum. The device weighs less than 500 grams and has a small form factor, as it
should be for mobile use.
The input stage of the 64 referential input channels to the amplifier has been optimized for high-
density EEG recorded with waveguard head caps. The preamplifier component implements active
shielding, which greatly suppresses external noise effects that typically contaminate EEG signals.
Active shielding works such that the buffered input signal (no amplification of signal amplitude but
impedance transformation) is put on the coaxial shield of an electrode lead inside the waveguard
cap. This way, electrical transmission of external noise through the coaxial shield into the lead is
minimized due to the nearly perfect “shorting” (zero voltage difference) between the lead and its
shield.
The input stage of the amplifier registers incoming signals against a common dedicated passive
reference electrode. The measured voltages thus all depend upon the quality of the reference signal
and it is highly recommended to pay attention to good preparation of the reference.
In order to measure physiological signals with high sensitivity, the human body acting as a source of
voltage differences and the amplifier ground (the electrical zero-point from which the operating
range is determined) have to be connected. This can be - for convenience - an electrode lead in the
waveguard cap: the model CA-200 defines AFz to be the ground (GND) position. Nonetheless, any
other accessible body part can be used to make the GND connection – with no effect on the
measured signal amplitudes. For a good suppression of common-mode signals, it may actually be
more suitable to place the GND at some distance from the scalp electrode. The cap therefore
provides access to the GND with a microcoax socket in the cap cable near the neck (note that there is
a second socket for connection of an EOG lead). See the cap datasheet and its user guide for more
information.
The GND connection actually implements a driven ground, i.e. a small voltage of ca. 2.5 Volts relative
to the internal amplifier ground is put on the human body. Electrical currents are kept well below
limits defined for patient safety in medical devices and order of magnitude smaller than currents
flowing in impedance measurements. The driven ground allows measuring signal amplitudes of the
voltage of the driven ground without saturation effects, regardless of the polarity of the signal. The
actual signal range is determined by the amplification factor of the input stage, which is
programmable in steps.
With the eego amplifier you can record bipolar signals using a referential setup and by re-referencing
in the eego software application. Please refer to corresponding documentation for more details how
to prepare the configuration with the appropriate adapter (XS-260).
Further you can record true bipolar signals via the dedicated bipolar input ports at the rear of the
housing. Two terminals per channel are available and must be connected.
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