The definition of SoC varies, and it is challenging to provide a precise definition due to its diverse content and broad application. In a narrow sense, it refers to the core chip integration of an information system, where essential components of the system are integrated onto a single chip. In a broader sense, SoC represents a micro-scale system. If the CPU is considered the brain, then SoC can be viewed as a complete system that includes the brain, heart, eyes, and hands. Academically, both domestic and international experts generally define SoCs as chips that integrate microprocessors, analog IP cores, digital IP cores, and memory (or off-chip memory control interfaces). These are typically custom-made or designed for specific applications.
In terms of hardware design, the main process of oscillometric blood pressure detection involves obtaining the pressure signal from the cuff, analyzing the pulse signal derived from it, identifying the positions corresponding to systolic and diastolic pressures, and ultimately acquiring the data. Traditionally, the sensor signal is amplified, low-pass filtered to extract the pressure signal, and then converted into digital form via an A/D converter before being sent to a microcontroller. The signal is then bandpass filtered again using another set of A/D converters. This basic structure is illustrated in Figure 1.
An A/D converter is used to transform analog signals—such as voltage, current, pressure, temperature, humidity, displacement, or sound—into digital data through a specific circuit. Before conversion, these signals must first be converted into voltage signals by various sensors. After conversion, the output can have 8-bit, 10-bit, 12-bit, or 16-bit resolution.
Thanks to the integration of a high-precision 16-bit Σ-Δ A/D converter, the reference voltage can be programmed (as low as 10mV), allowing direct A/D conversion without compromising accuracy or dynamic range. This eliminates issues like dynamic range changes, noise, and voltage offset caused by amplifiers, reducing the number of devices used and lowering implementation costs.
Since the Σ-Δ A/D converter supports differential input mode, differential signals from the sensor can be directly fed into the converter, theoretically achieving infinite common-mode rejection. This significantly reduces common-mode interference caused by mismatches in the preamplifier circuit.
Additionally, because the Σ-Δ A/D converter includes a low-pass filter during the conversion process, no prior filtering is necessary. The sensor can be directly connected to the A/D converter before digital filtering.
The ADμC848 integrates a standard constant current source, which can be adjusted via software programming. This allows for sampling a standard pressure output, followed by A/D conversion. Based on the conversion result, the constant current source can be adjusted until the desired output value is achieved, enabling automatic calibration of the device.
The improved hardware structure of the electronic sphygmomanometer is shown in Figure 2.
Software design follows the hardware processing stage, where the pressure curve in the cuff is obtained. The software first separates the pulse signal, removes interference points, fits the envelope curve, and identifies the average pressure. Finally, the systolic and diastolic pressures are calculated based on the coefficient.
A morphological filtering algorithm is introduced to separate the pulse signal. Since the pressure signal in the cuff resembles the pulse signal band, direct band-pass filtering can reduce signal amplitude and lower the signal-to-noise ratio, making subsequent processing more difficult. Morphological filtering helps extract the pulse signal effectively from a structural perspective. To enable real-time signal separation, the open operation is applied to all peaks in the original signal, and the original signal is compared with the processed one to obtain the separated pulse signal. Figure 3 shows the original signal, while Figure 4 displays the separated pulse signal.
To suppress interference and repair defective pulse waves, the reliability of each pulse wave is determined based on the relationship between the peak value and adjacent peaks. Considering that pulse wave amplitudes are not monotonic, the amplitude factor must also be taken into account. The specific method is detailed in relevant literature.
Envelope fitting is performed using weight information from each pulse wave. Due to the asymmetry of the resulting envelope, a third-order weighted least squares fit is employed. After fitting, the pressure value at the maximum point on the curve corresponds to the average pressure.
Finally, based on the average pressure, the appropriate amplitude coefficient is selected, and this coefficient is used to calculate the positions of systolic and diastolic pressures, thereby determining the final blood pressure values.
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