The characterization for A-to-D are:
1). Range (voltage +/-)
2). Accuracy (bits resolution)
3). Sample Speed (snapshot time)
4). Sample Frequency (repeat rate)
5). Circuit complexity (limitations)

The good news is that your application is not pressing the limits. Whatever you choose will probably deliver the performance you need.

Peter is correct that serial is good when the data is NOT sampled frequently. However, the final decision also must factor in circuit complexity. If the micro has no other function to perform and has no other use of its many I/O lines, you may still choose a parallel A-to-D converter. Lacking any other I/O requirements, you may also choose a microprocessor with less pins to reduce circuit size and cost, in which case a serial would be again useful.

For example, Microchip sells a PIC microprocessor very inexpensively that has an internal software/register run A-to-D capability. Its a crude detector but it could drive system costs down if that's important. The PIC serial communications may not be as easy as the 8051 though. I'm no PIC fan, but when circuit size, and cost are a concern, you have to examine their micros.

I've seen some really bad mistakes in choosing A-to-D converters (ADC) in projects. I've had design contractors tell me that one ADC would suffice in a design that I knew absolutely wouldn't. Otherwise brilliant designers can drop the ball in this area. The problem may be that the chip by its very nature, rests upon the boundary between analog and digital engineering knowledge. It takes an understanding of both to choose well in the more complex applications.

I always say that its worth a second or third look before committing if your sample speeds are very frequent and your accuracy demands are high. Some ADC chips perform terribly in a noisy DC Power environment affecting accuracy terribly. There are others that allow you to better screen that effect.

*** You may pay caraful attention to this if your micro is powered from the same battery you are attempting to measure. This can affect your ADC voltage reference approach.

Another issue that is important in more complex applications that are sometimes overlooked is the multiple input sampling mechanism. Most only sample and digitize one signal at a time. In some applications, you need a to measure several signals at the same time to get the accurate view. Some ADCs do this, most don't.

-Jay C. Box