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Front End Amplifier

The electrical signal derived from the electrodes is typically 1mV peak-peak. An amplification of about
1000´ is necessary to render this signal usable for heart rate detection. Realizing clean amplification of
the EKG signal with such high gain is no easy task specifically with the given situation that the human
body acts as a huge antenna that picks up a lot of noise including a dominant 50Hz/60Hz line frequency
noise. This has to be filtered by a strong post filter after amplification. Unfortunately any amplification will
amplify the noise voltages in addition to the desired EKG signal. In certain situations the noise can
completely override the EKG and render the amplified signal useless.

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The EKG Amplifier


This EKG signal at the output of INA321 is further amplified by OA0, one of the three integrated
operational amplifiers in the MSP430FG439. Figure 1-2 is the circuit diagram for the EKG amplifier front
end.
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EKG Sampling

The amplified EKG signal is internally fed to the on chip analog to digital converter ADC12 input channel
A1. The ADC12 samples the EKG signal with a sampling frequency fsample of 512 Hz. Precise sampling
period is achieved by triggering the ADC12 conversions with the Timer_A pulses. The Timer_A is clocked
by the ACLK generated from the 32.768 kHz low frequency crystal oscillator.
The fastest deflection in the EKG is in the 20ms range and happens at the QRS complex. It is important to
capture the QRS complex in its entirety for useful medical evaluation of the EKG waveform. Having a
sampling frequency of 512 Hz or sampling period of approximately 2ms captures atleast 10 samples at the
QRS complex and ensures that the QRS complex is fully digitized. The QRS complex also serves as a
definite indicator for every heart beat hence it is necessary to have it captured for heart beat rate
calculation. The heart beat rate itself is typically in the 60 to 200 beats per minute or about 3 to 4Hz.

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