Medical devices must consider high precision, accuracy, and energy efficiency.
TI Provides Measurable Analog Front-End Solution on a Single Chip

With growing interest in health, healthcare data such as step counts and sleep patterns can be easily measured via smartphones or smartbands. In addition, healthcare medical devices are emerging that measure non-contact thermometers, blood pressure, blood sugar, and oxygen saturation.

For medical devices, accurate and high-precision results, high energy efficiency, and a small size are important. Since they are often used continuously for 24 hours, they must be capable of long-term measurements with low power consumption. This is typically required in applications for electrocardiogram (ECG) and photoplethysmography (PPG).

An electrocardiogram (ECG) electrically measures cardiac activity by determining heart rate using electrical signals generated by muscle contractions. The circuit between the patient and the electrodes can be affected by factors such as patient movement, the electrode attachment status, and respiration. In ECG devices, the signal is inconsistent due to interference from power, changes in DC values resulting from electrode attachment, muscle movement, and variations in electrode DC voltage. These issues are resolved through the use of band-pass filters via digital signal processing.

Photoplethysmography (PMP) optically measures changes in blood vessel volume, reflecting the activity status of the heart and cardiovascular system. It is a method that non-vascularly measures blood oxygen saturation by irradiating the fingertips with light of two different wavelengths. Because light reaches the skin first before reaching the blood, it can be affected by the value.

TI provides analog front-end solutions capable of measuring electrocardiograms (ECGs) and photoplethysmography (PPE) on a single chip. The ADS129x series is available for ECG measurement. The ADS1291 is a solution that integrates protection, lead-off detection, and an ADC into a single chip, making it suitable for configuring one-lead ECG systems.

The ADS1292 has two built-in ADCs and can measure up to 6 leads, while the ADS1298 is a solution that implements up to 12 leads and a WCT node on a single chip.

Photoplethysmography can be implemented using the AFE44xx solution. Basically, it is equipped with an LED driver to illuminate the LED and a function to detect the current flowing when reflected light enters the photodiode. Since the signal entering the photodiode has a high frequency, the front end receiving the signal must have a wide bandwidth.

TI's AFE4403 features four RC filters, each of which can be switched on only when desired. The DC correction DAC and second-stage amplifier are located on the rear of the TIA. The AFE4404, however, has the DC correction DAC on the front. It was installed at the front to address the potential issue where the AFE4403's large DC component could prevent the TIA from amplifying the signal. The AFE4403 features a dynamic power saving mode that can reduce power consumption by 35–70%.