Analog-To-Digital and Digital-To-Analog Conversion

In order for analog devices (e.g., speakers, temperature sensors, strain gauges, position sensors, light meters, etc.) to communicate with digital circuits in a manner that goes beyond simple threshold triggering, we use an analog-to-digital converter (ADC). An ADC converts an analog signal into a series of binary numbers, each number being proportional to the analog level measured at a given moment. Typically, the digital words generated by the ADC are fed into a microprocessor or micro controller, where they can be processed, stored, interpreted, and manipulated. Analog-to-digital conversion is used in data-acquisition systems, digital sound recording, and within simple digital display test instruments (e.g., light meters, thermometers, etc.).

In order for a digital circuit to communicate with the analog world, we use a digital-to-analog converter (DAC). A DAC takes a number and converts it to an analog voltage that is proportional to the number. By supplying different numbers, one after the other, a complete analog waveform is created. DACs are commonly used to control the gain of an op amp, which in turn can be used to create digitally controlled amplifiers and filters. They are also used in waveform generator and modulator circuits and as trimmer replacements and are found in a number of process-control and auto calibration circuits. the ADC receives an analog input signal along with a series of digital sampling pulses.

Each time a sampling pulse is received, the ADC measures the analog input voltage and outputs a 4-bit that is proportional to the analog voltage measured during the specific sample. With 4 bits, we get 16 binary codes (0000 to 1111) that correspond to 16 possible analog levels (e.g., 0 to 15 V). In the digital-to-analog conversion figure, the DAC receives a series of 4-bit. The rate at which new binary numbers are fed into the DAC is determined by the logic that generates them. With each new binary, a new analog voltage is generated.

As with the ADC example, we have a total of 16 numbers to work with and 16 possible output voltages. As you can see from the graphs, both these 4-bit converters lack the resolution needed to make the analog signal appear continuous (without steps). To make things appear more continuous, a converter with higher resolution is used. This means that instead of using 4-bit binary numbers, we use larger-bit numbers, such as 6-bit, 8-bit, 10-bit, 12-bit, 16-bit, or even 18-bit numbers. If our converter has a resolution of 8 bits, we have 28 = 256 binary number to work with, along with 256 analog steps. Now, if this 8-bit converter is set up to generate 0 V at binary 00000000 and 15 V at binary 11111111 (full-scale), then each analog step is only 0.058 V high (1⁄256 × 15 V). With an 18-bit converter, the steps get incredibly tiny because we have 218 = 262,144 binary numbers and steps. With 0 V corresponding to binary 000000000000000000 and 15 V corresponding to 111111111111111111, the 18-bit converter yields steps that are only 0.000058 V high! As you can see in the 18-bit case, the conversion process between digital and analog appears practically continuous.