Analogue-to-digital converters are often classified according to the conversion processor or the conversion technique used to digitize the signal. Based on various conversion methodologies, common types of A/D converters include flash or simultaneous or direct-conversion A/D converters, half-flash A/D converters, counter-type A/D converters, tracking A/D converters, successive approximation type A/D converters, single-slope, dual-slope and multislope A/D converters and sigma-delta A/D converters. Each of the above-mentioned types of A/D converter is described in the following paragraphs.

The simultaneous method of A/D conversion is based on using a number of comparators. The number of comparators needed for n-bit A/D conversion is 2^n−1. One such system capable of converting an analogue input signal into a two-bit digital output is shown in Fig. 12.29. The analogue signal to be digitized serves as one of the inputs to each of the comparators. The second input for each of the comparators is a reference input, different for each comparator. The reference voltages to be used for comparators are in general V/2^n, 2V/2^n, 3V/2^n, 4V/2^n and so on. Here, V is the maximum amplitude of the analog signal that the A/D converter can digitize and n is the number of bits in the digitized output.

The half-flash A/D converter, also known as the pipeline A/D converter, is a variant of the flash-type converter that largely overcomes the primary disadvantage of the high-resolution full-flash converter, namely the prohibitively large number of comparators required, without significantly degrading its high-speed conversion performance. Compared with a full-flash converter of certain resolution, while the number of comparators and associated resistors is drastically reduced in a half-flash converter, the conversion time increases approximately by a factor of 2. For an n-bit flash converter, the number of comparators required is 2^n−1, while the same for an equivalent half-flash converter would be 2×2^(n/2).

It is possible to construct higher-resolution A/D converters with a single comparator by using a variable reference voltage. One such A/D converter is the counter-type A/D converter represented by the block schematic of Fig. 12.32. The circuit functions as follows. To begin with, the counter is reset to all 0s. When a convert signal appears on the start line, the input gate is enabled and the clock pulses are applied to the clock input of the counter. The counter advances through its normal binary count sequence. The counter output feeds a D/A converter and the staircase waveform generated at the output of the D/A converter forms one of the inputs to the comparator. The other input to the comparator is the analogue input signal. Whenever the D/A converter output exceeds the analogue input voltage, the comparator changes state. The gate is disabled and the counter stops. The counter output at that instant of time is then the required digital output corresponding to the analogue input signal.

In the counter-type A/D converter described above, the counter is reset to zero at the start of each new conversion. The D/A converter output staircase waveform always begins at zero and increases in steps until it reaches a point where the analogue output of the D/A converter exceeds the analogue input to be digitized. As a result, the counter-type A/D converter of the type discussed above is slow. The tracking-type A/D converter, also called the delta-encoded A/D converter, is a modified form of counter-type converter that to some extent overcomes the shortcoming of the latter. In the modified arrangement, the counter, which is primarily an UP counter, is replaced with an UP/DOWN counter. It counts in upward sequence whenever the D/A converter output analogue voltage is less than the analogue input voltage to be digitized, and it counts in the downward sequence whenever the D/A converter output analogue voltage is greater than the analogue input voltage. In this type of converter, whenever a new conversion is to begin, the counter is not reset to zero; in fact, it begins counting either up or down from its last value, depending upon the comparator output. The D/A converter output staircase waveform contains both positive-going and negative-going staircase signals that track the input analog signal.

The development of A/D converters has progressed in a quest to reduce the conversion time. The successive approximation type A/D converter aims at approximating the analogue signal to be digitized by trying only one bit at a time. The process of A/D conversion by this technique can be illustrated with the help of an example. Let us take a four-bit successive approximation type A/D converter. Initially, the counter is reset to all 0s. The conversion process begins with the MSB being set by the start pulse. That is, the flip-flop representing the MSB is set. The counter output is converted into an equivalent analogue signal and then compared with the analogue signal to be digitized. A decision is then taken as to whether the MSB is to be left in (i.e., the flip-flop representing the MSB is to remain set) or whether it is to be taken out (i.e., the flip-flop is to be reset) when the first clock pulse sets the second MSB. Once the second MSB is set, again a comparison is made and a decision taken as to whether or not the second MSB is to remain set when the subsequent clock pulse sets the third MSB. The process continues until we go down to the LSB.

Figure 12.35 shows a block schematic representation of a single-slope A/D converter. In this type of converter, one of the inputs to the comparator is a ramp of fixed slope, while the other input is the analogue input to be digitized. The counter and the ramp generator are initially reset to 0s. The counter starts counting with the first clock cycle input. The ramp is also synchronized to start with the first clock input. The counter stops when the ramp amplitude equals the analogue input. In this case, the counter count is directly proportional to the analogue signal. It is a low-cost, reasonably high-accuracy converter but it suffers from the disadvantage of loss of accuracy owing to changes in the characteristics of the ramp generator. This shortcoming is overcome in a dual-slope integrating type A/D converter.

The sigma-delta A/D converter employs a different concept from what has been discussed so far for the case of various types of A/D converters. While the A/D converters covered so far rely on sampling of the analogue signal at the Nyquist frequency and encode the absolute value of the sample, in the case of a sigma-delta converter, as explained in the following paragraphs, the analogue signal is oversampled by a large factor (i.e., the sampling frequency is much larger than the Nyquist value), and also, it is not the absolute value of the sample but the difference between the analogue values of two successive samples that is encoded by the converter.