What is Digital to Analog Converter (DAC)? (Explained)

What is Digital to Analog Converter (DAC)?

A Digital to Analog converter is a tool that comes with one single chip or Integrated Circuit that converts digital or binary codes into analog signals. This device can change abstract and binary numbers consisting of a set of 0s or 1s to a specific physical variable.

KEY TAKEAWAYS

• A Digital to Analog Converter transforms digital data before sending it to the amplifier into analog signals. It may also report sample rates and bit depths which however do not have any significant effect on the quality of sound produced.
• The varied features of the Digital to Analog Converter make it useful which include resolution, total harmonic distortion, monotonicity, maximum sampling rate, phase distortion and dynamic range.
• The performance level of these converters is typically determined by taking their static, time domain and frequency domain performances together.
• The Digital to Analog Converter offers the fastest way to convert digital signals to analog signals along with large binary inputs for easy implementation.

Understanding Digital to Analog Converter (DAC)

Computer signals can be classified broadly into two groups namely, digital signals and analog signals.

The electronics devices that deal with digital signals are called digital electronics such as:

• Logic Gates
• Microprocessors
• Flip-Flops
• Microcontrollers and more.

On the other hand, the devices that deal with analog signals are called analog electronics such as:

• Power switches
• Op-Amp and more.

Depending on the type of the devices, the electronics design typically needs one of these two signals to be converted from one to another more often than not.

Technically speaking, a Digital to Analog Converter converts the binary values represented as 1s and 0s into a continuous set of analog voltages.

This is actually done by following several different methods and each of these techniques has its own characteristic advantages and disadvantages.

Rudiments

A Digital to Analog Converter, just as the name signifies, is vested with the job of doing the necessary conversions of a digital input signal into an analog output signal.

A Digital to Analog Converter is commonly referred to as DAC but is also expressed in different other ways such as:

• D to A Converter
• D2A Converter
• DAC Converter
• D/A Converter and others.

This is actually an electronic device and a data converter that converts digitized music into analog sound output, digital video signals into analog images, and more.

Importance of Digital to Analog Converter

As you may know already, a computer is a binary machine and it operates in an analog world. Not all devices can understand these signals.

Therefore, a Digital to Analog Converter is required to convert the signals so that it can produce an output that can be understood easily by other devices.

An example will make things clear to you. Imagine that you are listening to a piece of music on a speaker from the YouTube channel by using your computer.

Here, the computer stores the audio signals or the values of the sound wave in binary form. If you want to play these signals back as sound output on a speaker, you will need analog signals.

This is because the diaphragm of the speaker vibrates due to the intensity of the analog signal. It is this vibration that produces the desired sound output.

In such situations, it is only a Digital to Analog Converter that will convert the digital audio signals to analog file so that it can be played on the speaker.

Since the modern era and computing are both digital in nature, there is an increased demand for digitized data which is why there is an increased demand for the Analog to Digital converters or ADCs.

However, you must keep in mind that in everyday life it is only analog signals that are used, and the world too is analog.

Therefore, whenever an Analog to Digital converter is needed, you will also require a Digital to Analog converter.

Features and Characteristics

The performance of the Digital to Analog Converter depends on its features and characteristics.

Here are some of the most important ones explained for you.

Resolution

This indicates the probable number of output levels that the Digital to Analog Converter is designed to produce and is expressed by the number of bits used by it.

This is actually the binary logarithm of the specific number of levels.

For example, a 1-bit Digital to Analog Converter is normally designed to produce 2 (21) levels but an 8-bit system will produce 256 (28) levels.

Resolution indicates the color depth in a video application as well as the depth of the audio bit in audio applications and is relative to the effective number of bits.

This is the measure of the actual resolution achieved by the Digital to Analog Converter.

Maximum Sampling Rate

This refers to the maximum speed at which the circuitry of the Digital to Analog Converter can operate and reproduce the correct output.

Usually, the relationship between the bandwidth of the sampling signal and the speed is defined by the Nyquist–Shannon sampling theorem.

Monotonicity

The Digital to Analog Converter is able to move its analog output specifically in the direction in which the digital input moves.

This means that before the correct output is asserted, it does not decrease even if the input is increased.

This feature is extremely helpful in situations where a DAC is used as a trim element that is digitally programmable or as a signal source of low frequency.

Total Harmonic Distortion and Noise

Often referred to as THD+N, this is a measure for the distortion and noise in the signal that is introduced by the Digital to Analog Converter.

This is actually expressed as the percentage of the total amount of power of the harmonic distortion and noise that are unnecessary but typically accompanies the preferred signal.

Dynamic Range

This is actually the measure for the difference between the largest signals and the smallest signals produced by the Digital to Analog Converter.

It is expressed in decibels and is typically associated with the noise floor and resolution.

The effective dynamic ranges of the Digital to Analog Converter are improved with the help of non-linear PCM or Pulse Code Modulation encodings such as A-law or μ-law, ADPCM or Adaptive Differential Pulse Code Modulation, and NICAM or Near Instantaneous Companded Audio Multiplex.

It uses logarithmic step sizes for the strengths of the output signals that are symbolized by each data bit.

This results in higher quantization distortion of loud signals which eventually helps the quiet signals to perform better.

Phase Distortion and Jitter

Some other characteristic features are also necessary to measure the performance of the Digital to Analog Converter such as phase distortion and jitter.

This is very important for those particular applications such as composite video and wireless data transmission that relies heavily on precise production of signals that are adjusted in different phases.

Performance Metrics

The static, frequency domain, and time domain performance of the Digital to Analog Converter is determined on different parameters with respect to each.

For example, the static performance is determined by:

• Differential Nonlinearity or DNL, which signifies the amount of deviation of two contiguous code analog values from the ideal one in 1 LSB or Least Significant Bit step
• Integral Nonlinearity or INL, which shows the amount of deviation of the Digital to Analog Converter transfer characteristic from the ideal one or the difference of the actual voltage for a particular code value from the ideal characteristic straight line in 1 LSB step
• Gain error
• Offset error and
• Noise, which is eventually limited by the thermal noise produced by the passive components in it such as the resistors.

In the case of audio applications in room temperatures, this noise is under 1 microvolt (μV) of white noise.

This typically restricts the performance to a level lower than 20~21 bits even in the 24-bit Digital to Analog Converters.

As for the frequency domain performance, it is determined by:

• Spurious Free Dynamic Range or SFDR, which signifies the ratio between the largest unwanted spur and the power of the converted central signal and is expressed in dB
• Signal-to-Noise and Distortion or SINAD, which signifies the ratio between the sum total of the noise and the harmonic spurs generated to powers of the converted key signal and is expressed in dB
• HDi or i-th Harmonic Distortion, which signifies the i-th harmonic of the converted key signal and
• Total Harmonic Distortion or THD, which indicates the sum total of the powers of all the harmonics in the input signal.

The Digital to Analog Converter is considered to be monotonic when the maximum DNL is below 1 LSB.

However, there are several monotonic Digital to Analog Converters that may have a maximum DNL in excess of 1 LSB or Least Significant Bit.

And, as for the time domain performance of the Digital to Analog Converter, it is measured by the glitch impulse area or the glitch energy.

The Circuit in a Digital to Analog Converter

Digital to Analog Converter comes with an Integrated Circuit which converts digital signal into analog current or voltage.

This is necessary for processing the analog signal further.

Converse to the circuits of the Analog to Digital converters, the Digital to Analog Converter has digital circuits that work with binary signals.

These signals typically have two distinct states such as:

• A logic ‘1,’ which signifies a High and
• A logic ‘0,’ which signifies a Low.

This means that the device needs to use an electronic circuit that will help in converting the two dissimilar domains of constantly changing analog signals and disconnected digital signals.

This is where the analog to digital converter comes into play.

It typically takes the snapshot of the analog voltage at a specific point in time and generates a digital code as output.

This actually represents the analog voltage.

The number of bits or binary digits used to signify this value of analog voltage is dependent on the resolution of the A/D converter.

Typically, in a Digital to Analog Converter the conversion of digital signals to analog signals simply refers to the weighted amount of the binary input. Therefore, a circuit called the summing amplifier is used.

This is actually an op-amp amplifier that comes with several resistors that are connected to a single input.

The point of intersection of these resistors is called the virtual ground or the summing junction.

The binary input enters the resistors and it gives an analog signal on the output of this op-amp.

Ideally, it is the resistors that make the circuit of the Digital to Analog Converter work.

That is why it is needed to be chosen correctly and matched perfectly so that an exact analog output can be achieved.

The higher the bits, the more different values of resistors are required but this may not be very realistic always.

As for the circuits of the Digital to Analog Converters, one of the most significant features of them is that even the nominal alteration defines the resolution.

Types of Digital to Analog Converter

There are different types of Digital to Analog Converters available that differ in design and performance. These are:

• PWM or Pulse Width Modulator Digital to Analog Converter – In this type of Digital to Analog Converter a steady voltage or current is switched into a low-pass analog filter. The duration of this switch is however decided by the digital input code.
• Oversampling or Interpolating Digital to Analog Converter – In this type of devices delta-sigma modulation is employed. This uses a pulse density conversion method with oversampling. These devices can attain a speed of more than a hundred thousand samples per second and offer a resolution of 24 bits.
• Binary Weighted Digital to Analog Converter – This type of device contains separate electrical components for every individual bit of the converter related to the summing point, which is, in general, an operational amplifier. This type of converter can offer only 8 bits of resolution and is also known for their poor accuracy due to high precision needed for every individual current or voltage. Every input in the summing point has values in powers of twos with most voltage or current and the Most Significant Bit. These devices are one of the fastest converters that produce accurate voltages or currents sum to the exact output value.
• Switched Resistor Digital to Analog Converter – This type of converter comes with a parallel resistor network wherein each resistor is either bypassed or enabled depending on the digital input.
• Switched Current Source Digital to Analog Converter – In this type of DAC different sources of current are selected depending on the digital input.
• Switched Capacitor Digital to Analog Converter – This type of DAC also comes with a parallel capacitor network where every capacitor is connected or disconnected depending on the input with switches.
• R-2R Ladder Digital to Analog Converter – This is a binary weighted DAC in which a repeating cascading structure is used for the R and 2R values of the resistor. This makes the output more precise because the matched resistors of equal values can be produced with relative ease. This is the simplest type of Digital to Analog Converter since there are only two resistor values required that are arranged in a ladder. The 2R resistor takes in the binary input and the output is given at the bottom of the ladder after using a set of quite complex math.
• Cyclic or Successive Approximation Digital to Analog Converter – These converters construct the output successively during every cycle and the individual bits of the digital input in these converters are processed in each cycle until the whole of it is accounted for.
• The Thermometer Coded Digital to Analog Converter – This type of DAC comes with an equal current-source sector or resistor for every probable value of the output. There will be 255 segments in an 8-bit thermometer Digital to Analog Converter while in a 16-bit thermometer DAC there will be as many as 65,535 segments. These converters are pretty fast in operation and come with an architecture that guarantees highest precision. However, this will be offered at the cost of multiple components that need high density IC processors for practical implementations and fabrication.
• Hybrid Digital to Analog Converter – These types of DACs come with a combination of several systems in a single converter. Almost all Digital to Analog Converter Integrated Circuits are of this type because it is difficult to get a high speed, low cost, and highly precise device in one single unit.
• The Segmented Digital to Analog Converter – This type of DAC follows the combination of the thermometer coded principle for the MSBs or Most Significant Bits and the binary weighted principle for the LSBs or Least Significant Bits. Ideally, a complete thermometer coded design will give 100% segmentation and a complete binary weighted design will give 0% segmentation. However, the combinations of the two principles results in a compromise between precision due to the thermometer coded principle and the number of current sources or resistors due to the binary weighted principle.

Today, most of the Digital to Analog Converters use a stable reference current or voltage to generate the output value.

On the other hand, the multiplying DACs typically use a variable input current or voltage as a conversion reference to produce the output.

This, however, puts added design limitations on the bandwidth of the conversion circuits.

Also, the modern high-speed Digital to Analog Converters comes with an interleaved structural design.

There are several DAC cores used in these devices in parallel and the performance of the combined DACs is enhanced since the output signals generated by these devices are typically combined in the analog domain.

This combination of the signals allows it to be performed either in the frequency domain or in the time domain.

Uses of a Digital to Analog Converter

Ideally both Digital to Analog Converters along with the Analog to Digital converters have contributed to the digital revolution.

Here are a few specific applications of the Digital to Analog Converters in particular:

• Digital signal processing is made much easier with the signals converted into binary. A good example of it is audio editing. DACs are used particularly in MP3 music players, CD players, DVD players, mobile phones, laptops, and more.
• Digital power supplies are much improved with the use of Digital to Analog Converters which changes the current or voltage of a power supply such as in the microcontrollers that are pretty slow to be included in a power supply control loop.
• Special types of standalone Digital to Analog Converters are also used in high-end hi-fi systems such as in modern digital USB speakers, sound cards, and in voice over IP communications.
• Video encoding or the video encoder systems use Digital to Analog Converters to process the video signals and send digital signals to the Integrated Circuits.
• Digital displays also use a Digital to Analog Converter typically in the graphic controllers to use a lookup table and produce signals that are sent to the analog output like the RGB signals in order to drive the display.
• A Digital to Analog Converter is also used for calibration of dynamic forms which, in turn, helps in improving the accuracy of the system.
• The motor controlling devices also use Digital to Analog Converters because volume control signals are needed to control the motor.
• Digital potentiometer also uses a Digital to Analog Converter to digitally control an analog signal.
• Mechanical Digital to Analog Converters are also used in the Selectric typewriter of IBM which helps in controlling the type-ball.
• Digital to Analog Converters are also used extensively in the modern communication systems to produce transmission signals that are defined digitally. For mobile communications high speed DACs are used and in the optical communication systems ultra-high-speed Digital to Analog Converters are used.

You will also find Digital to Analog Converters being used in software radio, data distribution systems and in several other places.

How Does a Digital to Analog Converter Work?

In order to understand the working process of the Digital to Analog Converter you will first need to understand the binary system.

This is actually a positional system or to put it in a much better way, a binary system is a place value system.

Here, every bit represents the absence or presence of a particular power of two in the total sum of powers.

This means that you can consider the whole process of digital to analog conversion to be a scaling operation.

In this process, mapping of a binary count is done to a specific voltage range with 0 volts as the minimum voltage and the highest input binary voltage as the maximum voltage.

Ideally, a digital signal is described in two ways such as:

• The discrete in time and
• The discrete amplitude signal.

On the other hand, an analog signal is described as:

• Continuous in time and
• Continuous in amplitude signal.

A Digital to Analog Converter typically converts the fixed-point binary number, which is also considered to be the abstract adequate precision number, into a physical quantity.

There are several steps involved in the transformation process which typically involves converting abstract data into theoretical series of impulses by the Digital to Analog Converter.

This series is then processed with the help of the reconstruction filter.

As said earlier, the Digital to Analog Converter works on the basis of the Nyquist-Shannon sampling theorem.

This particular theorem states that recovering the input signal from its sampled output can be done if the rate of sampling is twice greater than or equal to the largest frequency component that is there in the input signal.

This performance can be measured by using specific parameters such as the signal to noise ratio, the output signal bandwidth and others, as mentioned before.

What are the Advantages of Digital to Analog Converter?

The Digital to Analog Converter is as important as an Analog to Digital Converter and there are too many good reasons to say that.

However, just like any other electrical or electronic device, the DAC too has its characteristic merits and demerits.

Some of advantages offered by the Digital to Analog Converter are:

• It is one of the fastest means to convert digital signals into analog signals
• It allows converting large digital or binary inputs very easily and
• It is easy to implement due to its simple circuitries.

However, the Digital to Analog Converter too has some downsides such as the use of costly operational amplifiers by the circuit and higher power indulgence.

Also, the register in use within the circuit results in offset errors, gain errors, and nonlinearity.

Still, in spite of the downsides of the Digital to Analog Converters, the utility of these devices in modern computing and the digitized world is immense and cannot be overlooked.

Conclusion

As this article points out, you can now see how a Digital to Analog Converter is useful in ensuring that the I/O performance of the computer system is at a constant high level.

Even if one is not a technical person, gaining such knowledge is always good for any computer user.