Digital to analog converter
In electronics, a digital to analog converter (abbreviated to DAC or D-to-A) is a device for converting a digital (usually binary) code to an analogue signal (usually a current (electricity) or voltage). This is done with switches or a network of resistors or current sources.
An analog to digital converter performs the reverse operation.
An important characteristic of a DAC is its monotonicity. This basically says that as the values of the digital input increase, the analogue output never decreases. It is a measure of the accuracy of the DAC to do its job.
Applications
Audio
An analogue 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
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.
Signal Processing
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.
DAC Types
The most common types of electronic DACs are:
- the Pulse Width Modulator, the simplest DAC type. A stable current 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.
- the Delta-Sigma DAC, a pulse density conversion technique. A series of precisely timed pulses with an average density corresponding to the digital input value are filtered to produce an analog voltage or current which contains much less noise in the passband than Pulse Width Modulation. 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.
- 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.
See also