The fourth prototype amplifier consists of an Analog Devices AD623AN instrument amplifier IC and active 4th-order Butterworth lowpass filtration using a custom-made equal-component Sallen-Key filter. The lowpass cutoff for this amplifier is ~30 Hz, as this value is more appropriate for the acquisition of EKG as opposed to EEG signals, and it also reduces the amount of AC line noise acquired. The gain of the instrument amplifier chip is fixed at 1000x, although the equal-component Sallen-Key filter also adds a gain of about 2.5x, giving an overall gain of about 2500x.

Also in this design, the National Semiconductor MF4 discrete lowpass filter IC has been replaced with a custom-built continuous filter made from two op-amps and additional RC components. This replacement significantly improved the overall stability of the amplifier, and drastically reduced the inter-component noise caused by the switching of the discrete IC (and its power supply issues). All future designs will use custom continuous filters instead of turnkey ICs. At present, I strongly discourage the use of the MF4 or other discrete ICs in analog designs.

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 now AC coupled using bipolar 1u capacitors combined with the 100K gate current resistors. This provides a highpass cutoff (-3db, 1st order) at about 1.6 Hz, which is quite low enough for EKG 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 gain of the AD623 was fixed at 1000x, its maximum. According to the datasheet, this provides the maximum common mode rejection for up to 100 Hz. This is also about the right gain for EKG acquisition, depending on the electrodes and lead configuration used.
  
  
• The output 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. Currently, 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. Attenuation is used primarily to record the precordial (V1-6) EKG leads, since they have the largest magnitude. Attenuation can be reduced or removed in order to record the limb leads (I-III, aVR-F), if necessary.
  
  
• An unused IC socket is included in this design so as to allow the addition of either another input channel, or another stage of gain beyond 2500x.

Front side

Back side

Frequency response

©Copyright Sky Coyote, 2001-2002.