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.
