Hardware Development
Week 6 - Saturday
August 25th, 2018
Discussion:
The PPG signal has two components, frequently referred to as AC and DC. The AC component is mainly caused by pulsatile changes in arterial blood volume, which is synchronous with the heartbeat. So, the AC component can be used as a source of the blood glucose concentration with the heart rate data. This AC component is superimposed onto a large DC component, hence the DC component must be removed to measure the AC waveform with the high signal-to-noise ratio. Since the useful AC signal is only a very small portion of the whole signal, an effective amplification circuit is also required to extract desired information from it.
August 25th, 2018
Signal Conditioning Theory
Objective: To develop the IR detection circuit based on photoplethysmography (PPG) principle.Discussion:
The PPG signal has two components, frequently referred to as AC and DC. The AC component is mainly caused by pulsatile changes in arterial blood volume, which is synchronous with the heartbeat. So, the AC component can be used as a source of the blood glucose concentration with the heart rate data. This AC component is superimposed onto a large DC component, hence the DC component must be removed to measure the AC waveform with the high signal-to-noise ratio. Since the useful AC signal is only a very small portion of the whole signal, an effective amplification circuit is also required to extract desired information from it.
Figure 2 Signal conditioning circuit diagram
From the figure above, the sensor output is first passed through a RC high-pass filter (HPF) to get rid of the DC component. The cut-off frequency of the HPF is set to 0.7 Hz. Next stage is an active low-pass filter (LPF) that is made of an Op-amp circuit. The gain and the cut-off frequency of the LPF are set to 101 and 2.34 Hz respectively. Thus, the combination of the HPF and LPF helps to remove unwanted DC signal and high frequency noise including 60 HZ mains interference, while amplifying the low amplitude pulse signal (AC component).
Conclusion:
The output from the signal conditioning stage goes to the similar HPF/ LPF combination for further filtering and amplification. The two stages of filtering and amplification converts the input PPG signals and they are synchronous with the heart rate to measure the blood glucose concentration.