ADC basics and comparative analysis of different ADC technologies

This world is both analog and often cut into digital fragments by us. As communications became more digitized and computing power increased, signals were converted into numbers that were easier to transmit and mathematically easier to calculate. The signal conversion process is an analog-to-digital process, which has led to advent of analog-to-digital converter (ADC).

The digital signal is synchronized to clock frequency. According to Nyquist criterion, sampling rate of ADC should be at least twice maximum frequency of analog input being digitized. This frequency is known as Nyquist sampling rate.

Figure 1. ADC sampling rate

As shown in Figure 2, quantization granularity (grain) of an ADC increases with increasing sample rate and resolution. The y-axis is resolution, which is actually number of ADC bits. The x-axis represents sampling rate.

Figure 2. ADC accuracy depends on sample rate and resolution

The number of digits is defined as a power of 2. Therefore, an 8-bit ADC will have a resolution of 28 or 256 bits. For a simple example of a 3-bit ADC converter, its resolution is shown in ladder analogy of Figure 3, with y-axis showing 3-digit ADC code and x-axis showing signal voltage range and gain.

Figure 3: ADC digital code and input voltage

For those who are new to digital signals, you can look at it this way: using binary code "1" and "0" means simply treating a bit with a value of "1" as turning on a switch. , and a bit with a value of "0" as in a switch. It is very different from decimal number system based on number of fingers on our human hands. In electronic circuits, it is not easy to implement a ten-position switch. Consequently, focus was on switching transistors faster, resulting in a frenzy of high frequencies.

Several factors are taken into account when digitizing a signal. They cause errors such as quantization errors between voltage detection levels, and other errors include signal non-linearity.

The error is not limited to analog part of ADC. Noise and clock jitter also affect digital signals.

Different ADC types have different sources (of errors). The main types of ADCs are:

1. Flash ADC

2. Sigma-Delta ADC

3. Double Slope ADC

4. Successive approximation ADC

The choice of ADC is usually based on speed and accuracy. The more bits an ADC has, more accurate it will be, but slower it will be (because each bit adds one clock cycle). On fig. 4 compares speed and accuracy of various ADCs.

Figure 4. Comparison of different ADC technologies

Although ADCs are digital devices, a significant amount of analog technology is involved in interface of each ADC technology. Also, type of communication bus available can help in selection process, as many serial links have bandwidth or bit limits. As with any product, more performance and speed required, greater complexity and cost. This has led to an ever-expanding ADC market with ever-expanding applications, similar to sensor market. Similar to sensor market, with development of technology and improvement of functions, ADC is also constantly improved.