Design of Portable Low Power ECG Signal Acquisition System

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Overview:
The ECG signal is a bioelectrical signal generated by myocardial contraction and provides information on changes in cardiac physiological function. The measurement electrode is placed in different parts of the body, and the potential difference of different body surfaces is recorded, and the electrocardiogram is obtained. (Electro Cardio Gram, ECG). Because it is easy to detect and has good intuitiveness, it is widely used in clinical medicine. However, the traditional ECG signal acquisition device is large in size and is not convenient for obtaining ECG signals in real time. Therefore, it is important to study portable, low-power ECG signal acquisition systems. Based on the low-power analog front-end ADS1293, this paper combines the MSP430 series low-power single-chip microcomputer to design an ECG signal acquisition system that can be used for ultra-low power consumption and miniaturization.

1 system hardware design ECG signal acquisition system is mainly composed of signal acquisition front-end ADS1293 and MSP430 microcontroller control circuit. The working principle is as follows: the human body ECG signal extracted by the electrode is first sent to the ADS 1293 for proper amplification and then analog-to-digital conversion to become a digital signal. Then, it is sent to the MSP430 MCU through the SPI interface for analysis and processing, and finally sent to the portable display device to display the waveform in real time through the USB interface of the MCU.

Figure 2 ADS1293 schematic
(1) Signal input interface. There are 6 signal input pins from IN1 to IN6. All input pins contain an electromagnetic interference (EMI) filter to filter out RF noise. The system uses a 5-lead connection, ie the right arm (RA), left arm (LA) and left leg (LL) are connected to the IN1, IN2 and IN3 pins respectively; the common mode detector (CM detect) obtains RA, LA and The average voltage of the LL is used as the input to the right leg drive (RLD) amplifier, and the output of the right leg drive amplifier is returned to the right leg (RL) terminal, which is input from the IN4 pin. The function of the right leg drive circuit is to control the patient's common mode level and increase the AC rejection ratio of the system; the output of the Wilson Network (WCT) is connected to the IN6 pin, together with the V1 (thoracic electrode) connected to the IN5 pin as the CH3 channel. Differential signal input.
(2) Signal processing unit. It is mainly composed of three parts: instrument amplifier (INA), ∑Δ regulator (SDM) and low-pass digital filter (Digital Filter). Its main function is to convert differential analog voltage signals into digital signals. The instrumentation amplifier is an operational amplifier circuit with high input impedance. It has two main functions: 1) Amplifying the differential signal. 2) Provide high input impedance to obtain a larger input signal from the ECG electrode. The output signal of the instrument amplifier is sent to the ∑Δ modulator for analog-to-digital conversion. The ∑Δ modulator is a one-bit coding technique based on oversampling, and the output reflects one bit of the encoded data stream of the input signal amplitude. The low-pass digital filter consists of three programmable 5th-order sin-type filters. The output of the ∑Δ modulator is processed by a low-pass digital filter to obtain an N-bit coded output.
(3) Signal output interface. It mainly includes 4-wire SPI serial interface, DRDYB pin and ALRAM pin. The functions of each pin are as follows: SCLK is the serial clock input pin; SDI is the serial data input pin, a total of 16 bits, of which 1 bit read and write control, 7 bit address and 8 bit data. During the rising edge of the clock, all data is sampled and written to the register on the 16th rising edge of the clock. SDO is a serial data output pin that reads data on the SDO pin on the falling edge of 8 to 15 clocks. CSB is a chip select pin, active low. During low time, the SPI interface starts reading and writing data, and the low level is maintained for 16 clock cycles. DRDYB is the analog to digital conversion end pin, indicating that the internal data of the chip is ready. Readable, active low, usually used as interrupt signal or status inquiry signal of CPU; ALARMB is alarm pin, ADS1293 has a self-diagnosis alarm system for diagnosing abnormal conditions that may occur in ECG applications. Abnormal conditions mainly include electrode dropout, synchronization error, low-power warning and abnormal operation of three channels. When an abnormal situation occurs, it is reported to the error flag and displayed on the ARLAM pin.

1.2 MSP430 MCU control circuit selects TI's ultra-low-power MCU MSP430F5529 as the main controller, which has a wealth of on-chip peripherals, each module runs completely independent, including timer, input/output port, watchdog and UART. It can run independently while the main CPU is in a sleep state. The typical value of current is 290μA/MHz when all modules are active. In standby mode, the typical value of current is only 0.18μA, and the response time from standby to wake-up is 3.5μs. The MSP430F5529 contains two Universal Serial Communication Interface (USCI) modules that support multiple communication modes such as UART, IrDA, I2C, SPI, and USB. In the system, the MSP430F5529 uses the SPI interface to control and transfer data to the ADS1293. The MSP430F5529 operates in master mode and the ADS1293 operates in slave mode. The data is transmitted to the portable display device or computer via the USB interface. Figure 3 shows the interface circuit of the MSP430F5529.