What is the test for validity of a search warrant obtained by affidavits allegedly tainted with illegally obtained evidence?

This circuit has developed a two-pronged test for the validity of search warrants obtained by affidavits allegedly tainted with illegally obtained evidence. First, the defendant must establish a factual nexus between the illegality and the challenged evidence. Once the defendant meets this burden, the burden shifts to the government to show that there was independent probable cause for the warrant to issue. United States v. Alexander, 761 F.2d 1294, 1299 (9th Cir.1985); United States v. Spetz, 721 F.2d 1457, 1468 (9th Cir.1983).