Also what kind of circular saw are you using. A table saw will stop the splintering as the wood is supported close to where the blade cuts. A hand held saw needs to be adjusted so the blade only just cuts through the material and a fine cut blade is beneficial as well. You can make a biscuit jointer with an angle grinder on instructables
I'm using a hand held circular saw. The blade is not what you would call "fine". I've reduced the depth of the cut, but will try to reduce it a bit more.