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.

Schematic

Notes

• Neither the power supply, nor the chip bypass, components are shown. See the power supply page for schematics. The AD623 is bypassed by two capacitors: a 100n film, and a 10u. The op-amps are bypassed by a single 100n capacitor.
  
  
• The op-amps used in this implementation (power supply and filter) are OP295s instead of OP213s. I don't think this makes any difference. Any decent single-supply op-amp should work fine. However, I usually prefer to use Analog Devices ICs if possible.
  
  
• The input is AC coupled using bipolar 1u capacitors combined with 100K gate current resistors. This provides a highpass cutoff (-3db, 1st order) at about 1.6 Hz, which is quite low enough for EEG signals. The input can be DC coupled, although depending on the ambient electrical environment this may preclude any acquisition of a signal due to supply rail clipping. The input to the second stage amplifier chip is also AC coupled using a 1u BP capacitor and a 100K resistor. Since the negative input to the second stage is connected to V0, the second stage amplifier is essentially single-ended, and the use of an instrument amplifier IC is perhaps overkill.
  
  
• The gain of the first AD623 was fixed at 1000x, its maximum. According to the datasheet, this provides the maximum common mode rejection for up to 100 Hz.
  
  
• The intermediate signal is filtered by an active 4th-order Butterworth lowpass equal-component Sallen-Key filter. The basic design for this filter was taken from Don Lancaster's Active Filter Cookbook, which I highly recommend. This book can be ordered from Amazon for under $30. The cutoff for this filter is ~30 Hz, and the damping of the two 2nd order stages is 1.848 and 0.765. This filter provides about another 2.5x in gain.
  
  
• The output is referenced with respect to V0. The output is attenuated using a voltage divider constructed from 2K and 500 Ohm resistors. This is because while the output of the amplifier can be up to 9 VPP (or whatever the battery happens to be that day), the sound input of the Mac clips at 1.5 VPP.
  
  

Front side

Back side

Frequency response

©Copyright Sky Coyote, 2001-2002.