A federal judge yesterday granted limited immunity to the former administrative assistant of Sen. Herman E. Talmadge (D-Ga.), clearing the way for the aide to testify before the Senate Select Committee on Ethics.

U.S. District Court Judge John Lewis Smith signed the order granting "use" immunity to Daniel Minchew, whose testimony the committee has been seeking concerning a secret bank account in Talmadges name at the Riggs National Bank.

Smith signed the order in a brief hearing yesterday after receiving a sealed envelope from the Justice Department, which had delayed the immunity grant for 20 days. The federal law giving Congress the power to grant immunity from prosecution concerning matters covered in testimony before a congressional committee also given the Justice Department the 20-day delay in order to document evidence it may later wish to produce against the witness.

The Justice Department, as part of its own investigation, has subpoenaed the Riggs accounts of Minchew and his wife, Shirley, Talmadge has acknowledged that his name appeared on a secret account of nearly $40,000 that was active in the Riggs bank in 1973 and 1974. Talmadge has emphatically denied knowing about the bank account until told by his staff.

Minchew, who left Talmadge's staff in 1974 to become a member of the International Trade Commission, is expected to appear before the committee today, to have the grant of immunity formally conference upon him and then to begin interviews with the committee staff before giving testimony under oath.