Digital-to-analog converter
In
electronics, a digital-to-analog converter is a device for converting a digital code to an analog signal . Digital-to-analog converters are the interface between the abstract digital world and the analog real life. Simple switches, a network of
resistors,
current sources or capacitors may implement this conversion.
An analog-to-digital converter performs the reverse operation.
A DAC usually only deals with
pulse-code modulation -encoded signals. The job of converting various compressed forms of signals into PCM is left to codecs.
Encyclopedia
In
electronics, a
digital-to-analog converter is a device for converting a digital code to an analog signal . Digital-to-analog converters are the interface between the abstract digital world and the analog real life. Simple switches, a network of
resistors,
current sources or capacitors may implement this conversion.
An analog-to-digital converter performs the reverse operation.
A DAC usually only deals with
pulse-code modulation -encoded signals. The job of converting various compressed forms of signals into PCM is left to codecs.
Basic ideal operation
The DAC fundamentally converts finite-precision numbers into a physical quantity, usually an electrical voltage. Normally the output voltage is a
linear function of the input number. Usually these numbers are updated at uniform sampling intervals and can be thought of as numbers obtained from a sampling process. These numbers are written to the DAC, sometimes along with a clock signal that causes each number to be latched in sequence, at which time the DAC output voltage changes rapidly from the previous value to the value represented by the currently latched number. The effect of this is that the output voltage is
held in time at the current value until the next input number is latched resulting in a
piecewise constant output. This is equivalently a
zero-order hold operation and has an effect on the frequency response of the reconstructed signal.
The fact that practical DACs do not output a sequence of
dirac impulses but instead output a sequence of piecewise constant values or
rectangular pulses, means that there is an inherent effect of the
zero-order hold on the effective frequency response of the DAC resulting in a mild roll-off of gain at the higher frequencies . This zero-order hold effect is a consequence of the
hold action of the DAC and is
not due to the sample and hold that might precede a conventional analog to digital converter as is often misunderstood.
Applications
Audio
Most modern audio signals are stored in digital form and in order to be heard through speakers they must be converted into an analog signal. DACs are therefore found in
CD players,
digital music players, and PC
sound cards.
Specialist stand-alone DACs can also be found in high-end hi-fi systems. These normally take the digital output of a
CD player and convert the signal into a line-level output that can then be fed into a
pre-amplifier stage. Some of these can also be made to interface with computers using a
USB interface.
Video
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 , 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 analog signal digitally.
DAC types
The most common types of electronic DACs are:
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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.
- Oversampling DACs 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 noise, thus pushing this noise out of the pass-band. Most very high resolution DACs 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 Binary Weighted DAC, which contains one resistor
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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 R-2R Ladder DAC, which is a binary weighted DAC that uses a repeating cascaded structure of resistor values R and 2R. This improves the precision due to the relative ease of producing equal valued matched resistors . However, wide converters performs slowly due to increasingly large RC-constants for each added R-2R link.
- the Thermometer coded DAC, which contains an equal resistor or current source segment for each possible value of DAC output. An 8-bit thermometer DAC would have 255 segments, and a 16-bit thermometer 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.
- the Segmented DAC, which combines the thermometer coded principle for the most significant bits and the binary weighted principle for the least significant bits. In this way, a compromise is obtained between precision and number of resistors or current sources . The full binary weighted design means 0% segmentation, the full thermometer coded design means 100% segmentation.
- Hybrid DACs, 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.
DAC performance
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:
- Resolution: This is the number of possible output levels the DAC is designed to reproduce. This is usually stated as the number of bits 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 which is a measurement of the actual resolution attained by the DAC.
- Maximum sampling frequency: This is a measurement of the maximum speed at which the DACs circuitry can operate and still produce the correct output. As stated in the Shannon-Nyquist sampling theorem, a signal must be sampled at over twice the bandwidth
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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 internal resampling.
- monotonicity: 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.
- THD+N: This is a measurement of the distortion and noise introduced to the signal by the DAC. It is expressed as a percentage of the total power of unwanted harmonic distortion and noise that accompany the desired signal. This is a very important DAC characteristic for dynamic and small signal DAC applications.
- Dynamic range: This is a measurement of the difference between the largest and smallest signals the DAC can reproduce expressed in Decibels. 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.
DAC Figures of Merit
- Static performance:
- DNL shows how much two adjacent code analog values deviate from the ideal 1LSB step
- INL 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 .
- Gain
- Offset
- Frequency domain performance
- SFDR indicates in dB the ratio between the powers of the converted main signal and the greatest undesired spur
- SNDR 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 indicates the power of the i-th harmonic of the converted main signal
- THD is the sum of the powers of all HDi
- Time domain performance
- Glitch Energy
- Response Uncertainty
- TNL
See also
Links and books
- See an Audio Note valve DAC kit with USB interface being built.
- with circuit diagrams.
- explains how INL and DNL are calculated.
- 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.
