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Former NFL QB Troy Smith was arrested for driving impaired and possession of marijuana near his old Ohio State stomping grounds early Sunday morning.

According to cops in Westerville, OH ... Smith was weaving in lanes in his Lincoln MKZ and when officers pulled him over, they noticed his eyes were glassy and his speech was slurred.

Cops say Smith denied he'd been drinking. When they put the Heisman winner through field sobriety tests ... he lost his balance 3 times, couldn't put his feet heel to toe, and when asked to recite the alphabet from C to W ... he stopped at I.

The officers said they smelled booze and marijuana on Smith ... so they searched his vehicle and found a glass jar with two "rolled cigars containing marijuana."

Smith also blew a .143 during a breathalyzer test at the police station.

He was booked for OVI (operating vehicle while impaired), marijuana possession, and failure to display a front license plate -- then released without bail to his girlfriend.