An Analog-to-digital Converter (ADC) performs the reverse operation.
A DAC usually only deals with Pulse-code Modulation (PCM)-encoded digital signals. The job of converting various compressed forms of signals into PCM is left to Codec s.
It is very difficult to construct a perfect DAC, as a mathematically correct reconstruction of the value of the encoded analog signal at a given point in time requires knowing not only all past, but also all future sampled values of the signal. So e.g. before a piece of music could be perfectly played from a CD, the perfect player would need to read it entirely into memory, which is not practical. In mathematical terms, a perfect DAC is ''non- Causal '' and proper digital to analog conversion requires applying a Convolution . Therefore, various simplified techniques exist, which strive to properly ''fill in the blanks between samples''. The most common one is to first produce an imperfect analog signal and then filter it, to make it closer to the original signal.
Most modern audio signals are stored in digital form (for example MP3 s and CDs ) and in order to be heard through speakers they must be converted into an analog signal. DACs are therefore found in CD Player s, Digital Music Player s, and PC Sound Card s.
Video signals from a digital source, such as a computer, must be converted to analog form if they are to be displayed on an analog monitor. As of 2003 , analog monitors are more common than digital, but this may change as flat panel displays become more widespread. The DAC is usually integrated with some Memory ( RAM ), which contains conversion tables for Gamma Correction , contrast and brightness, to make a device called a RAMDAC .
A device that is distantly related to the DAC is the Digitally-controlled Potentiometer , used to control an analogue signal digitally.
The most common types of electronic DACs are:
- the , the simplest DAC type. A stable Current (electricity) or Voltage is switched into a low pass Analog Filter with a duration determined by the digital input code. This technique is often used for electric motor speed control, and is now becoming common in high-fidelity audio.
- such as the '''Delta-Sigma DAC''', a pulse density conversion technique. The Oversampling technique allows for the use of a lower resolution DAC internally. A simple 1-bit DAC is often chosen as it is inherently linear. The DAC is driven with a Pulse Density Modulated signal, created through Negative Feedback . The negative feedback will act as a High-pass Filter for the Quantization (signal Processing) noise, thus pushing this noise out of the pass-band. Most very high resolution DACs (greater than 16 bits) are of this type due to its high Linearity and low cost. Speeds of greater than 100 thousand samples per second and resolutions of 24 bits are attainable with Delta-Sigma DACs. Simple first order Delta-Sigma modulators or higher order topologies such as '''MASH - ''''''Multi stage'''''' Noise SHaping ''' can be used to generate the pulse density signal. Higher Oversampling rates relax the specifications of the output Low-pass Filter and enable further suppression of quantization noise.
- the , which contains one Resistor or Current Source for each bit of the DAC connected to a summing point. These precise voltages or currents sum to the correct output value. This is one of the fastest conversion methods but suffers from poor accuracy because of the high precision required for each individual voltage or current. Such high-precision resistors and current-sources are expensive, so this type of converter is usually limited to 8-bit resolution or less.
- the , which is a binary weighted DAC that creates each value with a repeating structure of 2 resistor values, R and R times two. This improves DAC precision due to the ease of producing many equal matched values of resistors or current sources, but lowers conversion speed due to parasitic capacitance.
- the , which contains an equal resistor or current source segment for each possible value of DAC output. An 8-bit binary segmented DAC would have 255 segments, and a 16-bit binary segmented DAC would have 65,535 segments. This is perhaps the fastest and highest precision DAC architecture but at the expense of high cost. Conversion speeds of >1 billion samples per second have been reached with this type of DAC.
- , which use a combination of the above techniques in a single converter. Most DAC integrated circuits are of this type due to the difficulty of getting low cost, high speed and high precision in one device.
DACs are at the beginning of the analog signal chain, which makes them very important to system performance. The most important characteristics of these devices are:
- : This is the number of possible output levels the DAC is designed to reproduce. This is usually stated as the number of Bit s it uses, which is the base two Logarithm of the number of levels. For instance a 1 bit DAC is designed to reproduce 2 () levels while an 8 bit DAC is designed for 256 () levels. Resolution is related to the '''Effective Number of Bits''' ( ENOB ) which is a measurement of the actual resolution attained by the DAC.
- '''Maximum , a signal must be sampled at over twice the Bandwidth of the desired signal. For instance, to reproduce signals in all the Audible Spectrum , which includes frequencies of up to 20 kHz, it is necessary to use DACs that operate at over 40 kHz. The CD standard samples audio at 44.1 kHz, thus DACs of this frequency are often used. A common frequency in cheap computer Sound Cards is 48 kHz - many work at only this frequency, offering the use of other sample rates only through (often poor) internal Resampling .
- : This refers to the ability of DACs analog output to increase with an increase in digital code or the converse. This characteristic is very important for DACs used as a low frequency signal source or as a digitally programmable trim element.
- ''' Distortion and noise that accompany the desired signal. This is a very important DAC characteristic for dynamic and small signal DAC applications.
- '''s. This is usually related to DAC resolution and Noise Floor .
Other measurements, such as Phase Distortion and Sampling Period Instability , can also be very important for some applications.
- Static performance:
- --- DNL (Differential Non-Linearity) shows how much two adjacent code analog values deviate from the ideal 1LSB step
- --- INL (Integrated Non-Linearity) shows how much the DAC Transfer Characteristic deviates from an ideal one. That is, the ideal characteristic is usually a straight line; INL shows how much the actual voltage at a given code value differs from that line, in LSBs (1LSB steps).
- --- Gain
- --- Offset
- Frequency domain performance
- --- SFDR (Spurious Free Dynamic Range) indicates in dB the ratio between the powers of the converted main signal and the greatest undesired spur
- --- SNDR (Signal to Noise and Distortion Ratio) indicates in dB the ratio between the powers of the converted main signal and the sum of the noise and the generated harmonic spurs
- --- HDi (i-th Harmonic Distortion) indicates the power of the i-th harmonic of the converted main signal
- --- THD ( Total Harmonic Distortion ) is the sum of the powers of all HDi
- Time domain performance
- --- Glitch Energy
- --- Response Uncertainty
- --- TNL (Time Non-Linearity)
- S. Norsworthy, Richard Schreier, Gabor C. Temes, ''Delta-Sigma Data Converters''. ISBN 0-7803-1045-4.
- Mingliang Liu, ''Demystifying Switched-Capacitor Circuits''. ISBN 0-750-67907-7.
- Behzad Razavi, ''Principles of Data Conversion System Design''. ISBN 0-780-31093-4.
- Phillip E. Allen, Douglas R. Holberg, ''CMOS Analog Circuit Design''. ISBN 0-195-11644-5.
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