A319 Captain departing ramp at CYYZ is instructed to turn right on Taxiway A but turns early remaining on ramp.

Narrative

Upon clearance from ground to departure ramp on Taxiway A; First Officer and I discussed and reviewed diagram and agreed that the first right turn was Taxiway A. I then turned toward this first right and it was the inner ramp taxiway instead of Taxiway A. We had briefed planned taxi out; I had the Dispatcher out and First Officer had detailed with Ramp. I looked for taxiway signs and saw 'A' signage that appeared to indicate inner ramp and 'A' and could not locate any pavement markings or signs on other taxiways -- also looked to see how many taxiways were between us and first runway and still thought inner was 'A.' Just as First Officer called right side clear and I turned nosewheel; Ground asked us to stop and contact ramp. We were recleared by Ramp to continue via ramp and then exit at next point to 'A.' In spite of all the preparation and discussion; I still initiated a turn thinking it was 'A.' Improved signage would help; but we also discussed after airborne that several of the airports we operate in and out of have only an inner and outer (e.g.; ORD). Callback conversation with Reporter revealed the following information: Reporter informed the analyst that the airport in question was CYYZ.