** Nordic Semiconductor NOD.OL falls as much as 6.6% after customer Dexcom DXCM.O filed for U.S. approval of a Silicon Labs-based chip, fuelling concerns it may no longer rely solely on the Norwegian firm's Bluetooth chips

** Analysts see Dexcom adding a second supplier for Bluetooth Low Energy chips that Nordic currently makes, and expect the move will hit revenues

** "Dexcom regulatory filing in our view confirms intent to start dual sourcing," DNB Carnegie analyst Christoffer Wang Bjørnsen says in a note

** Skandinaviska Enskilda Banken analyst Markus Heiberg says Texas Instruments TXN.O, which bought Silicon Labs, plans to make older chips in the U.S., giving Dexcom a U.S.-made option

** DNB Carnegie and SEB see any hit to Nordic's sales starting around mid-to-late 2026 and extending into 2027

** Shares were down 4.6% at 0840 GMT, among the top fallers on the STOXX 600 .STOXX in a mixed European chip sector