In electronics, a digital-to-analog converter (DAC or D-to-A) is a device for converting a digital (usually binary) code to an analogue signal (current, voltage or charges). Digital-to-Analog Converters are the interface between the abstract digital world and the analogue real life. Simple switches, a network of resistors, current sources or capacitors may implement this conversion.
An analog to digital converter or ADC performs the reverse operation.
An analog signal from a microphone or other sound source can be converted to digital form for storage in a computer, where it can be edited if necessary and then reconstructed for playback. In a personal computer, the conversion is usually done in a sound card, but there are some USB devices that do this conversion externally to improve the sound quality.
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.
DACs are used in analogue signal processing circuits to replace potentiometers. They allow small adjustments to be made to the circuit by software, instead of the old technique of using a screwdriver. Because this type of DAC is updated only infrequently, it often has a slow serial interface. Some types have non-volatile memory to enable them to remember their last settings when the power is switched off.
The most common types of electronic DACs are:
- 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 (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 >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 relaxes the specs of the output Low-pass filter and enables further suppression of quantization noise.
- the Binary Weighted DAC, 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 R2R Ladder DAC, 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 Segmented DAC, which contains an equal resistor or current source segment for each possible value of DAC output. An 8-bit binary Segmented DAC would have 256 segments and a 16 bit binary Segmented DAC would have 65536 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.
- 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.
DACs are usually the used as the final output interface between computers and other digital systems and continuous analog circuitry. Being at the beginning of the analog signal chain makes then very important to system performance. There are many important characteristics of DACs that help a designer to choose the best one for his project.
- DACs are usually first classified by their Resolution. This is the number of possible output levels the DAC is designed to reproduce. This is usually stated as a binary number of bits (you may calculate the actual level number by raising 2 to the number of bits). 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.
- Another important DAC characteristic is the maximum sampling frequency. This is a measurement of the maximum speed at which the DACs circuitry can operate and still produce the correct output. It is very important to match the sampling frequency of your DAC to the signal you are trying to reproduce. The Shannon-Nyquist sampling theorem states that you must sample at twice the frequency of the highest output frequency you wish to reproduce.
- One more important characteristic of a DAC is its 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 is another important DAC characteristic. THD+N is a measurement 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 is also very important for DACs. 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.
DAC Figures of Merit
- 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 characterisitc deviates from an ideal one; (others: Gain and 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 ration between the powers of the converted main signal and the sum of the noise and the genrated 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 HD;
- Time domain performance: Glitch Energy; Response uncertainty (TNL - Time Non-Linearity);