The concept of System on Chip (SoC) varies widely, making it challenging to define precisely due to its extensive content and broad application areas. In a narrow sense, an SoC is the core chip that integrates essential components of an information system onto a single chip. In a broader context, it refers to a micro-scale system. If the central processing unit (CPU) is considered the brain, then the SoC can be seen as a complete system that includes the brain, heart, eyes, and hands. Academically, both domestically and internationally, SoCs are generally defined as chips that integrate microprocessors, analog IP cores, digital IP cores, and memory (or off-chip memory control interfaces). These are typically custom-designed or tailored for specific applications, rather than standard products.
In the hardware design of an oscillometric blood pressure detector, the main process involves acquiring the pressure signal from the cuff, analyzing the pulse signal derived from it, identifying the positions corresponding to systolic and diastolic blood pressure, and finally obtaining the data. Traditionally, the sensor signal is amplified, low-pass filtered to extract the pressure signal, and then sent to a microcontroller via an A/D converter. The pressure signal is further bandpass filtered to isolate the pulse signal, which is then converted again using another set of A/D converters. The basic structure is illustrated in Figure 1.
An Analog-to-Digital (A/D) converter transforms analog signals into digital ones through a specific circuit. Analog inputs can include electrical signals like voltage or current, or non-electrical signals such as pressure, temperature, humidity, displacement, and sound. Before conversion, these signals must be converted into voltage signals by sensors. After A/D 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), enabling direct A/D conversion without compromising accuracy or dynamic range. This eliminates issues like dynamic range variation, noise, and voltage offset caused by amplifiers, reduces device usage, and lowers 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.
Because the Σ-Δ A/D converter inherently filters the signal through a low-pass filter, there's no need for additional filtering before A/D conversion. The sensor can be directly connected to the A/D converter, followed by digital filtering.
The ADμC848 integrates a standard constant current source, allowing software-programmed adjustment of the current. Based on the product’s environment, a standard pressure output can be sampled, converted, and the constant current adjusted accordingly until the desired output is achieved, enabling automatic calibration of the device.
The improved hardware structure of the electronic sphygmomanometer is shown in Figure 2.
In the software design phase, after the hardware processing, the pressure curve within 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 coefficient is used to calculate the systolic and diastolic pressures.
A morphological filtering algorithm is introduced during the pulse signal separation process. Since the pressure signal in the cuff resembles a pulse signal, direct bandpass filtering may reduce the signal amplitude and lower the signal-to-noise ratio, complicating subsequent processing. Morphological filtering helps separate the signal based on its shape, effectively extracting the pulse signal. To achieve real-time processing, the open operation is applied to all peaks in the original signal, and the processed signal is compared with the original 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 distorted pulse waves, the reliability of each pulse wave is determined by comparing the peak value with adjacent peaks. Since the pulse amplitude isn't always monotonic, the amplitude factor must also be considered. The detailed method is outlined in relevant literature.
Envelope fitting is performed using the weighted information from each pulse wave. Given the asymmetry of the resulting envelope, a third-order weighted least squares fit is used. Once the fitting is complete, the pressure 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 determine the positions of systolic and diastolic pressures, thereby calculating the final blood pressure values.
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1.0mm Female Header
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