The COEX pins are powered from VDD_GPIO (pin 12). They have the same voltage level as VDD_GPIO and any unwanted noise or spurious that VDD_GPIO may contain. Therefore, proper noise filtering in VDD_GPIO is important. It is also important for each individual COEX pin to meet the filtering requirements specified for the peripherals connected to them.
To reduce transient currents, series resistors or ferrite beads can be used in the COEX lines. For low current, digital control type of COEX routings, resistor values in the range of 100 Ω to 1 kΩ can be considered. However, the optimal resistor values depend mainly on the connected peripheral drive currents and communication speed required by the peripherals.
In addition to the series resistor, bypass capacitors in the range of few pF can be considered. They may help to reduce RF coupling to the COEX lines. Unused COEX pins can be left unconnected electrically, but it is recommended to connect them to the application board for improved thermal and mechanical performance. For more information on the COEX interface configurations, see nRF9160 Product Specification.