Get-Aduser -Filter will not accept a variable

I'd like to check if a user account already exists in the system.

$SamAc = Read-Host 'What is your username?'$User = Get-ADUser -Filter {sAMAccountName -eq "$SamAc"}

I'm not sure why, but $User will always return null even if {sAMAccountName -eq "$SamAc"} is supposed to be true.

What am I missing here?

Edit:

This is what was missing:

$User = Get-ADUser -Filter "sAMAccountName -eq '$SamAc'"

^{Editor's note: The script block ({ ... }) was replaced with a string.}