Stock index futures turned negative on Wednesday as doubts set in that a large take-up at the European Central Bank's three-year funding operation will ease a regional credit crunch.

S&P 500 futures dipped 3.1 points and were below fair value, a formula that evaluates pricing by taking into account interest rates, dividends and time to expiration on the contract. Dow Jones industrial average futures lost 1 point, and Nasdaq 100 futures fell 2.25 points.

(Reporting By Angela Moon; editing by Jeffrey Benkoe)