3 Design

A field-applicable design is to add a filter to the SCLK line to remove noise. Since SPI communication itself is not particularly fast, the possibility of additional filters affecting communication due to delays is very low. A low-pass filter is required to remove high frequencies. The cutoff frequency should be approximately three times the operating frequency and margin, taking into account the operating frequency. Figure 3-1 shows the LPF basic characteristic curve.

Figure 3-1 Low Pass Filter Characteristic Curve

Low pass filter (LPF) cutoff frequency is F_c. F_c expressed by the following equation:

Furthermore, the filter must be placed as close to the slave, AFE4xxxx, and pin as possible to eliminate clock noise and maintain normal device operation. Placing the filter in the middle of the communication line or on the master side can result in noise re-introducing itself into the filtered signal. Figure 3-2 shows how to place the filter.

Figure 3-2 Proposed LPF Schematic into SCLK Path

Another approach is to increase the hysteresis margin of the SCLK buffer when designing the next version of the device. The trend in wearable devices is to minimize external noise to achieve power consumption, product size, and price competitiveness. Therefore, improving the hysteresis of buffers in developing next-generation products is something that must be considered not only for wearable devices but also for all devices targeting PE, and this performance increases the competitiveness of the product.