1.2 Data converters  (Page 31/3)

Technical overview

A data converter is the bridge between the real, physical world of analog signals like voltage or current, and the digital world of numbers represented by ones and zeros. An analog-to-digital converter (ADC) converts a voltage into a number; a digital-to-analog converter (DAC) converts a number into a voltage or current. An ADC might be used to measure a weight or the intensity of light, or allow an audio signal to be captured and stored as a digital file for playback in a media player. Converting that digital file back into sound would require a DAC; a DAC can also control a valve that affects the flow of chemicals into a chemical reaction, or the position of a cutting head on a system that makes mechanical parts.

ADCs

Figure 1 is a general representation of an ADC. An analog-to-digital converter can be represented by the three functional blocks shown here, regardless of architecture.

Every ADC consists of three functional blocks: a sampler, a quantizer and an encoder. In some architectures, some of the functions may actually be combined, but each function is there nonetheless.

The sampler is responsible for sampling the input signal at a certain time; it is implied that this function also “holds” the signal constant for the converter to operate on it during its conversion time.

The quantizer is responsible for measuring the input signal and determining an output code level that most closely represents the voltage of the analog input. It approximates the sampled voltage with a level from a fixed set of 2 ^N possible voltage levels (where N is the number of bits of resolution), either via rounding or truncation.

The measurement of the input signal and creation of its corresponding output code are accomplished by comparing the input signal to a fixed reference voltage. The full-scale range (the maximum voltage that the converter can have on its input) is directly related to the reference voltage value. The minimum change in input voltage that the converter can detect is called the least significant bit (LSB) value. For example, if the full-scale range is the same as the 5-V reference voltage V _REF , and the converter has 12 bits of resolution, the LSB would be given by Equation 1:

LSB = V _REF /2 ^N = 5 V/2 ¹² = 5 V/4096 = 1.22 mV (1)

You can generally achieve the best conversion accuracy by matching the input signal range closely to the converter's full-scale range, either through amplification before the ADC or by changing the reference voltage to adjust the full-scale range.

The encoder can turn the internal code used by the quantizer into a more usable code for a system (for example, turning a thermometer code into a twos complement code) or can simply format the code into a serial data stream for easy communication to a host processor.