1 Introduction

The AFE4xxx family is a versatile signal acquisition system that supports acquiring PPG, EDA, ECG, and bioimpedance signals. A PPG signal chain comprises a current driver for LEDs and a receiver that senses the signal from PDs. An ECG receiver with a right leg drive (RLD) can acquire a single-lead ECG from a pair of electrodes. An impedance measurement signal chain with integrated excitation and sense can be used for applications such as BIA, EDA, and impedance spectroscopy. The outputs of the various signal chains are digitized by a common ADC and stored in a FIFO, which can be read out using an SPI interface.

In increasingly compact wearable devices, the signal integrity (SI) of each signal is a critical factor. Recent wearable devices are susceptible to noise because the devices incorporate various communication modules, such as Bluetooth, Wi-Fi, and LTE, in addition to internal wired communication. As a result, minimizing the coupling and noise levels that can occur within the application to prevent malfunctions is crucial. This report describes countermeasures for SPI communication issues and the situations that must be considered during product design to mitigate the impact of such noise.

Figure 1-1 shows the basic SPI communication pins and connections. SPI 4-wire communication consists of four communication lines: SCLK, SDIN, SEN, and SDOUT. Furthermore, separate pull-up and down is not required, and direct connections between the master and slave are required.