Re: Band-gap ref for ADC
First of all: If your supply is also 3.3V, then it may be complicated to get the desired 3.3 reference for the ADC, maybe you can get 3.1V or so. and if you need 3.3V when your supply is 3.3V-10%, then you have a much bigger problem. If your supply is larger than 3.3V, then forget about all this.
Now, about the error. It is true, for 14bits, you need errors in the order of 100uv or less. That is definitively a challenge for a voltage reference and a buffer (which is probably what you need). But, I think, and I'm not sure here that you may have some sort of calibration to remove offset errors, so you first have to know which contributions are really critical for performance and then take decisions about architecture.
So, first create the right specs, then do the rest.