Digital-to-Analog Converter (DAC)

It is a device that converts a digital code (usually binary, made up of zeros and ones) into an analog signal (current, voltage or electric charge). Different components can be involved in this process, such as simple switches, resistor networks, current sources or capacitors. An analog-to-digital converter (ADC) performs the reverse operation.

BASIC CHARACTERISTICS

The main parameters that define a digital-to-analog converter are, first of all, its resolution, which depends on the number of input bits of the converter. Another basic characteristic is whether it is capable of unipolar or bipolar conversion. A third is the code used for the input information -digital-to-analog converters generally operate with natural binary code or with binary-coded decimal (BCD)-. Conversion time is another feature that defines the converter required for a given application. It is the time the converter needs to carry out its maximum voltage change with minimum error in its resolution. Other aspects of the converter are: its reference voltage, which may be internal or external (if external, it can be varied within certain limits); the output voltage will be affected by this factor, which is implemented by means of a multiplying converter; likewise, the supply voltage, the temperature range and the internal technology must also be taken into account.

USES

Most modern audio signals are stored digitally (MP3s and CDs, for example), and in order to be heard through loudspeakers they must be converted to analog. CD players, digital music players and PC sound cards therefore all have a device of this type built in.

A standalone DAC (we review one in this issue of CEC) can also be found as a separate chassis in hi-fi systems. These separate DACs take the digital output from a CD player (or a dedicated transport) and convert the signal before sending it to the amplifier. Some of them can be connected to personal computers using a USB interface. In fact, Josep Armengol's review of the Moon 300D DAC explains how to do it.

DAC SPECIFICATIONS

There is a wide variety of DACs available, either as integrated circuits or as encapsulated packages. You need to know the manufacturers' most important specifications in order to evaluate a DAC for a particular application.

- Resolution: The percentage resolution of a DAC depends solely and exclusively on the number of bits. For this reason, it is stated this way in data sheets. A 10-bit DAC has a finer resolution (greater accuracy) than an 8-bit one. This also applies to the specifications of CD players or integrated components.

- Accuracy: DAC manufacturers have several ways of stating precision or accuracy. The two most common are called Full-Scale Error and Linearity Error (or sometimes simply linearity), which are normally expressed as a percentage of the converter's full-scale output (%FS).

Full-scale error is the maximum deviation of the DAC's output from its expected (theoretical) value.

Linearity error is the maximum deviation in step size from the theoretical step. Some of the less affordable DACs have full-scale and linearity errors in the range of 0.01% - 0.1%.

- Settling time: The operating speed of a DAC is expressed as settling time, which is the period required for the output to go from zero to full scale when the binary input changes from all zeros to all ones. Typical settling time values range from 50 ns to 10 ms. In general, DACs with current output will have shorter settling times than those with voltage output. For example, a DAC may operate with either current or voltage output. Its output settling time is shorter when current output is used than when voltage output is used.

- Offset voltage: In theory, the output of a DAC will be zero volts when the binary input is all zeros. In practice, there will be a small output voltage produced by the offset error of the DAC's amplifier. This offset is commonly 0.05% FS. Almost all voltage-output DACs have an external offset adjustment that allows this imbalance to be eliminated.

Although manufacturers' figures often do not match reality, at least we now know what a digital-to-analog converter is for and how it works. The goal, as always at CEC, is for readers to form their own judgment when it comes to searching for or buying a DAC.

DAC APPLICATIONS

DACs are used whenever the output of a digital circuit has to provide an analog voltage or current to drive or activate an analog device. Some of the most common applications are described below:

- Control: The digital output of a computer can be converted into an analog control signal to adjust the speed of a motor or to control almost any physical variable.

- Automated testing: Personal computers can be programmed to generate the analog signals (through a DAC) needed to test analog circuits. The analog output response of the circuit under test will normally be converted into a digital value by an ADC.

- Digital amplitude control: A multiplying DAC can be used to digitally adjust the amplitude of an analog signal. Remember that it generates an output that is the product of a reference voltage and the binary input. If that voltage is a time-varying signal, the DAC's output will follow this signal, but with an amplitude determined by the binary input code. A typical use of this application is digital "volume control", where the output of a digital circuit or computer can adjust the amplitude of an audio signal.

- A/D converters: Several types of A/D converters use DACs as part of their circuitry.