EKG Signal Conditioning for the ADC
Notes:
The cycling computer uses the analog-to-digital converter to sample the EKG signal that appears across the handlebar contacts when they are gripped.
Short leads connect the handlebar contacts to an instrumentation amplifier that multiplies the 4 mV peak-to-peak EKG signal by 1980. Input capacitors C1 and C2, along with shunt resistors R1 and R2, precede the instrumentation amplifier and form a high pass filter with a cut off frequency of 7.2 Hz.
After amplification, the signal is passed through a fourth order Chebyshev low pass filter that provides 90 dB of attenuation per decade beyond the cut off frequency of 10 Hz. This, along with the input high pass filter, eliminates extraneous noise beyond the 5 to 10 Hz frequency band of the signal that is digitized by the MC68HC705P6A.
After amplification and filtering, the EKG signal should lie approximately between -2.25 and +2.25 volts. A final stage in the signal conditioning chain provides a DC offset of 2.5 volts, placing the EKG waveform in the 0 to 5 volt range required by the MC68HC705P6A analog-to-digital converter.