Amplifier 5

The fifth prototype amplifier is designed for use in acquiring EEG signals, which have considerably lower input levels than do EKG signals. The basic design of this amplifier is the same as that of amplifier 4. In addition, another instrument amplifier chip has been added to increase the overall gain sufficiently to acquire EEG signals.

This amplifier consists of two Analog Devices AD623AN instrument amplifier ICs, and active 4th-order Butterworth lowpass filtration using a custom-made equal-component Sallen-Key filter. The lowpass cutoff for this filter is still at ~30 Hz (the same value as was used to acquire EKG signals), which may be somewhat low for EEG signals. Nevertheless, this filter band is wide enough to begin to acquire interesting and useful EEG signals (such as the alpha band), although the cutoff frequency may be increased to about 100 Hz in future designs.

The gain of the first instrument amplifier chip is fixed at 1000x, the equal-component Sallen-Key filter adds a gain of about another 2.5x, the gain of the second instrument amplifier IC is set at about 8x, and the output attenuator drops the signal by about 0.2, giving an overall effective gain of about 4000x. This is perhaps slightly small, delivering an output signal of about 0.25 VPP depending on the electrical contacts used and the strength of the underlying scalp signal. In future it may be desirable to increase the second stage amplifier gain to about 16x or more, in order to expand the output signal to the full range of the Mac sound input port.



Front side

Back side

Frequency response

The gain and phase response of this circuit (actually, of amplifier 4) at frequencies of interest (~2-80 Hz) is shown here. The frequency response of external electronics can now be quickly and easily acquired using the Mac and component software instead of an external oscilloscope.

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