Analog to digital (A-D) converters with a fixed conversion time are subject to errors due to metastability. These errors will occur in all converter designs with a bounded time for decisions, and are potentially severe. We estimate the frequency of these errors in a successive approximation converter, and compare the results with asynchronous designs using both a fully speed-independent, and a bundled data approach. It is shown that an asynchronous converter is more reliable than its synchronous counterpart, and that the bundled data design is also faster, on average, than the synchronous design. We also demonstrate tradeoffs involved in asynchronous converter designs, such as speed, robustness to delay variations, circuit size and design scalability.