The following is an excerpt from page 18 of Summary Appeal Law and Trial Advocacy discussing the basis for an acquittal appeal:
There Is No Such Thing as an “Unreasonable Acquittal”
There is no such thing as an unreasonable acquittal. This is a very important point which must be borne in mind. The defence can appeal a conviction as being unreasonable; however, the Crown cannot. This rule makes sense. The learned authors Sopinka and Gelowitz, in The Conduct of an Appeal, explain:
Analytically, therefore, the question whether there is an absence of evidence to support an acquittal is entirely separate from the question of an absence of evidence to support a conviction. A conviction based upon no evidence involves a fabrication of an element or elements of the offence; an acquittal upon no evidence is simply an application of the presumption of innocence. In other words, an acquittal is the default finding in a criminal trial where the Crown fails to discharge its burden of proof, positive evidence supporting an acquittal is never required.
 R. v. Labadie,  O.J. No. 1257, 275 C.C.C. (3d) 75 (Ont. C.A.).
 R.S.C. 1985, c. C-46.
  O.J. No. 3167, 2008 ONCA 589 (Ont. C.A.).
 Ibid., at para. 71 [emphasis added].
 R. v. Schuldt,  S.C.J. No. 76 (S.C.C.). However, the Crown can appeal an acquittal if the acquittal is based on conjecture. If there was no other rational conclusion based on the evidence other than the guilt of the accused then an acquittal would be based on conjecture and raise a question of law — see R. v. Wild,  S.C.J. No. 69 (S.C.C.) printed page 10 and Sopinka & Gelowitz, The Conduct of an Appeal, Third Edition (Toronto, ON: LexisNexis Canada, 2012) at paras. 3.19-3.23.
 Ibid., at para. 3.16.