They can be managed.

First of all, don't use the MAX232, but, rather, use the MAX232A or MAX3232 and its scions. Since they use smaller cap's and higher frequency, it is easier to bypass the switching noise they generate with more or less standard decoupling/bypass methods. If you want a really quiet supply network on your circuit, you have to have the contiguous low-impedance ground plane I mentioned, and you must keep your bypass members attached with leads as short as possible, preferably on the order of 1 mm. Your voltage reference MUST be carefully regulated and supplied with a ferrite-bead-isolated supply. For high precision, you'll need very good protection of the reference supply. For high accuracy, you'll need very careful bypass of Vcc so that you don't introduce circuit noise into your MCU's internals.

If you consult the documentation your MCU manufacturer provides, you'll likely see a number of methods for keeping the ADC precision relatively high. Don't be surprised if you still lose as much as three bits of your precision. You probably don't have an instrument with which you can detect the imprecision anyway.

RE