What are Digital to Analog converters?

Digital to analog converters are commonly known as DAC, digital analog converter, D2A, or Digital-to-Analog converters. DACs are one of the important devices used in the electronics and electrical domain. DACs are the systems that convert a digital signal into an analog signal, whereas an analog to digital converter or ADC performs the opposite function. Digital signals are those signals that can represent the information or data as a set of discrete values. At a particular instant of time, a digital signal can only take a particular value of data. A digital signal is characterized by two unique values; zeros and ones, or true and false, represented by the Boolean domain. These are also known as binary signals or logic signals. Unlike digital signals, an analog signal is represented by continuous values, where at any time instant, the analog signal is represented by a real number.

The digital signal has a different context of definition in different areas of science like in digital electronics, digital signals are a fixed width squared electrical pulse, each of these pulses occupies a discrete amplitude value. The binary signals which occupy either a low value or a high value are an excellent example of such signals in digital electronics. In digital signal processing, a digital signal is a representation of a sampled and quantized physical signal, the value of the signal only exists in regular intervals of time. The digital signal in this case can be stored, processed, and even transmitted. In digital communications, a digital signal is a continuous-time signal, which fluctuates between a discrete number of waveforms.

Digital to Analog Converters (DAC)

DACs are the system that converts a fixed binary number into a physical quantity such as voltage, acceleration, pressure, etc. DACs are used to convert finite time-varying data into continuously varying physical signals. There are multiple applications of DACs such as an audio amplifier, microphone, loudspeaker, and so on.

The architecture of a DAC

The working of DACs usually follows two methods.

Weighted resistor method

The DACs using the weighted resistor method are also known as resistor DAC. The DAC in this category is characterized by the ability to provide inputs that will eventually correspond to the contributions of different bits that are an integral part of the digital input. It means, if the input signals are in the voltage domain, the corresponding binary bits can be added by inverting the summing amplifier. An operational amplifier or op-amp is used in the weighted resistor method. The resistance of the input resistors which are connected with the op-amp has its values weighted in the form of binary bits. When the receiving binary bit is 1, the switch makes a connection with the resistor and the reference voltage. Upon receiving a binary value of 0, the switch grounds the resistor. As the number of bits increases, the accuracy of such DAC declines, due to an increase in the range of resistors.

R-2R ladder DAC

This DAC makes use of a repeated cascaded arrangement of resistors of resistance values R and 2R. These resistors are usually referred to as precision resistors. The word 'ladder' signifies the ladder configuration of the network of resistors which forms the R-2R configuration. These circuits provide a more simplified means of converting a digital input voltage into an equivalent analog voltage output. The resolution of the R-2R ladder network depends on the number of input points that receive the input voltages. The voltage-output signals are acquired from the other end of the ladder, which is fed into the op-amp that performs the inversion of input to get the desired equivalent output. The resistors in this case act as voltage dividers, whose output solely depends on the interaction of the input voltages.

Application of DACs

DACs are mainly used in digital signal processing applications and other applications. The various applications of DACs are outlined here in this section.

Audio amplifiers

In audio amplifiers, DACs are extensively used to produce DC voltage gain. The DC voltage gains in this case are produced by commands given to the microcontrollers. The audio codec holds the entire DAC and provides signal processing features. For instance, the Pro-Ject audio system provides excellent audio solutions using state of art DAC technology.

Video encoders

The integrated circuits (ICs) of video encoders have inbuilt DACs. The video encoders process the video signals and send them to different DACs to generate analog video signals of required formats.

Data acquisition systems

These devices measure real-time mixed-signals and are multichannel rather than begin a single-channeled. The data which is to be made digital is converted by the analog converters- (AC) which are sent to a processor for processing. The acquisition system has a process control feature that sends feedback to the system for efficient signal conversion.

Motor control DACs

The motor control DACs feature an external DAC, which is placed outside the motor periphery. These kinds of DACs are ideal for controlling voltage signals. The DACs in this category are operated by a microprocessor.

Display electronics

Many display electronics such as monitors, television screens, projectors, and so on, use a video DAC to produce analog outputs such as red, green, and blue (RGB) to drive the displays.

High precision Analog converters- the delta-sigma DAC

These are the fastest and high precision DACs. These DACs further consists of the following parts:

Interpolation filter

This part increases the sampling frequency by four times. Hence the output sampling rate is increased and thus the time required for sampling is reduced.

Modulator

It is preferably known as the delta-sigma modulator. It acts as a high pass filter for the noise and a low pass filter for the signal.

1-bit DAC

This part converts each bit of the signal individually into an analog output.

Analog output filter

This module generally filters the output from the DAC to produce a pure analog signal.

Context and Applications

This topic is widely taught in many undergraduate and postgraduate degree courses of:

• Bachelors of Technology (Electronics Engineering)
• Bachelors of Technology (Electronics and Electrical Engineering)
• Bachelors of Technology (Electrical Engineering)
• Master of Technology (Signals and Systems)
• Master of Technology (Electronics Engineering)

Practice Problems

Q1. Which of the following amplifiers is used in the weighted resistor DAC?

a. Op-amp

b. Headphone amp

c. Differential amplifier

d. None of these

Answer: Option a

Explanation: The weighted resistor DAC features an operational amplifier or op-amp. When the input bit is 1, the switch of the amplifier makes a connection with the resistor, and when the input bit is 0, the switch grounds the resistors.

Q2. Which of the following is the correct abbreviation for DACs?

a. Digitized analog converters

b. Digital to analog converters

c. Digital amplification converters

d. None of these

Answer: Option b

Explanation: The correct abbreviation for DACs is digital to analog converters. These are the systems that convert digital signals into analog signals which are continuous signals characterized by real numbers.

Q3. Which of the following applications make use of digital to analog converters?

a. Audio amplifiers

b. Data acquisition systems

c. Both a and b

d. None of these

Answer: Option c

Explanation: Audio amplifiers make use of the digital to analog converters to produce output voltage gains and are produced by commands processed by the microcontroller. In the data acquisition systems, the data to be made digital are processed by processors, the system has a feedback feature that interacts with the processor for efficient digital to analog conversions.

Q4. Which of the following modules of the delta-sigma DAC is used for sampling the frequency?

a. 1 bit DAC

b. Analog output filter

c. Modulator

d. Interpolation filter

Answer: Option d

Explanation: The interpolation filter produces a four-timed sampled output frequency. This module increases the sampling rate and reduces the time required for sampling.

Q5. Which of the following uses an external DAC?

a. Motor controller DAC

b. Display electronics

c. Video encoders

d. Audio amplifiers

Answer: Option a

Explanation: The motor controller DACs houses an external DAC which is mounted outside the motor periphery. These DACs are used to control the voltage signals used to control motor speeds.