Fundamentals of Data conversion

I really like a excellent summary about data conversion:

"No matter which ADC and what kind of ADC architecture, they all perform two basic and fundamental operations: discretization in time and discretization in amplitude. "

The first step, time discretization, is also called sampling operation. The continually time-varying input analog signal is sampled at uniformly spaced times at a frequency of fs and the samples are thus separated by a period of T=1/fs.

Although the sampled signal are obtained, they are still has different infinite range of values in amplitude domain. Therefore, we can't be represent them precisely in digital codes.

The second step is to digitize the sampled signal in amplitude. The so-called quantization error limits the resolution of the converter.

There are two broad categories of ADCs: Nyquist-rate and over-sampling converters. They typically offer different compromises between ADC resolution and output sampling rate.

Nyquist-rate converters are those that operate at the minimum sampling frequency necessary to capture all the information about the entire input bandwidth, and therefore the output data rate of a Nyquist-rate ADC can be very high.

Over-sampling converters are sampling at very high frequency. The noise-shaping technique is used to filter the in-band noise and to achieve higher SNDR and SFDR. Two types of over-sampling converters are developed currently, DT and CT. Indeed CT sigma delta ADC is introduced earlier in 1962 than DT sigma delta ADC. But when switched-capacitor circuits were introduced, most CT sigma delta modulators were implemented with DT loop filters. SC circuits remain popular because of their insensitivity to signal waveform characteristics.

In addition, the time constants of SC integrators scale with sampling frequency, allowing for greater system flexibility. However, interest in CT sigma delta modulators has been renewed because of their benefits versus sampled-input ADCs, such as employing lower-power integrator amplifiers and including inherent anti-aliasing filtering.

Because of the over-sampling operation, the output rates of sigma-delta ADCs are currently limited to less than 100MSPS while pipleine ADCs are capable of operating up to 500MSPS and beyond. Indeed, for a given technology, Nyquist-rate converters will always be able to operate faster than sigma-delta ADCs because of the over-sampling necessary in a sigma-delta design. Fortunately, the benefts of CT sigma delta technology outweigh the drawbacks for high-resolution applications at sampling rates below 100MSPS.