|Digital to analog converter|
|Wednesday, 06 December 2006|
A digital-to-analog converter (DAC) is configured to convert a digital signal into an analog voltage. The DA converter essentially includes a plurality of direct current voltage sources having generally different voltage levels, and a decoder portion having a plurality of switches for switching these voltage sources. A typical digital-to-analog converter includes an encoder, a number of analog output elements and a summing circuit. The encoder receives a digital input, which is a digital value represented by a number of binary data bits, and then encodes the binary data bits into suitable drive signals to selectively activate the analog output elements. In response to the drive signals, the activated analog output elements generate partial analog signals. These partial analog signals are then combined by the summing circuit to produce an analog output, which is an analog representation of the digital input. A DAC is typically implemented in an integrated circuit or chip, although it can be implemented on a circuit board by an appropriate arrangement of components. Digital-to-analog converters which are formed as integrated circuits typically are formed by mass-production processes. Two categories of digital-to-analog converters, the resistor and capacitor DAC's, are frequently used for such DA conversion. Resistor DAC (RDAC) are relatively simpler to make but consume static power. Electric power is consumed across the resistor of the RDAC even as the converter is in static status. By contrast, capacitor DAC (CDAC) are inherently more complex to fabricate, and their operation require more precise timing control, but they consume virtually no static power, which is an important characteristics of advantage in portable electronic devices. Capacitor digital-to-analog converters are used frequently in high precision analog-to-digital converter applications. Capacitors may be integrated and have less dependence on temperature than resistive circuits, especially thin-film resistors. Capacitors suffer other drawbacks, however, and require special care in order to provide reasonable speed and good tracking. MOS (metal-oxide-silicon) capacitors have relatively large stray capacitances to the substrate through their inversion layer depending on the oxide thickness. Double poly capacitors offer better performance, but the manufacturing process is far more complicated and expensive, with greater chances for errors. Digital-to-analog converters using capacitor integration must also be designed to tolerate stray capacitance.
Conversion from digital-to-analog (D/A) is the process of converting digital codes into a continuous range of analog signal levels. A digital-to-analog converter having weights using the power of two is called a binary weighted DAC. Resistor-string DACs are DACs that produce a voltage output. Current-steering DACs are DACs that produce analog current outputs. Current-steering DACs are implemented using an array of matched current sources which are unity decoded or binary weighted. There are a variety of architectures in which a current-steering DACs may be implemented from two-stage, interpolated, or segmented architectures. In a current mode digital-to-analog converter, each of the bits in a digital signal is provided to a switch that controls current flow in a separate branch of the DAC. In general, a current-mode DAC consists of an array of current sources that are individually switched on or off in response to a control input. The current source outputs can be combined to yield a total current that is proportional to the number of switched-on current sources. Currents are switched to outputs or ground by switches. The output currents are transformed into voltages by resistors and amplifiers. When a bit provided to one of the switches is "on", the switch is closed, thereby enabling current to flow in the respective branch. A segmented DAC design converts digital codes to analog signals by activating a number of weighted segments proportional to the input digital code and summing the activated segments to form the analog output signal. A current mode DAC generates a differential current output that is typically applied to a current-to-voltage converting amplifier to produce a differential voltage output. Current-mode DACs are widely used as they are well suited for driving resistive loads, they tend to consume less power than other alternatives, and they offer reasonable static linearity. Current mode digital-to-analog converters are popular in high-speed application. Digital codes are typically converted to analog voltages by assigning a voltage weight, or current weight, to each bit in the digital code and summing the voltage or current weights of the entire code. A digital-to-analog converter having weights using the power of two is called a binary weighted DAC. Binary weighted DACs assign appropriate weights in the current, voltage, and charge formats to the binary signals, and arrange the weighted binary signals to generate desired analog signals. DACs that produce analog current outputs usually have a faster settling time and better linearity than those that produce a voltage output. A single resistor string voltage scaling DAC produces an analog voltage from a digital word by selectively tapping a voltage-divider resistor string connected between a high and a low voltage reference voltage. A dual resistor string DAC, generally referred to as a two-stage cascaded converter, converts a digital word into a corresponding analog signal employing two-cascaded resistor strings. A constant impedance resistor string is comprised of 2N series-connected resistors, where N equals the number of bit of the digital word to be converted. A voltage reference is placed across the string to thereby generate a series of monotonically increasing voltages. The value of the digital word determines which one of these voltages is selected as the analog output.
The digital-to-analog converters are classified as audio DACs and video DACs. Voice, audio and video signals, information closely related to our daily life, are inherently analog in form. Contemporary technologies have, however, employed digital form for the processing and storage of these most-used information. Real world applications which use digital-to-analog converters (DACs) include, for example, digital audio systems such as compact disc players, digital video players, and various other high performance audio applications, which include conversion of digital signals to analog waveforms at a high resolution. A compact disc (CD) player retrieves sound information stored on a CD in digital format for playback. Before this high-fidelity music can be played back, however, its retrieved digital representation must be converted into analog electrical signal that can be used to drive a loudspeaker system. Good digital-to-analog conversion is crucial to high-fidelity music playback. Digital video electronic devices are designed to output video signals in various analog formats. In this way, various analog video displays are able to present the output video signals using any one of the supported analog formats. For example, an LCD (liquid crystal display) panel of a computer system also requires similar digital-to-analog conversion. The LCD panel is the instrument for presenting visual information to the operator/user of the computer. Information in the form of either text, graphics or video to be displayed on the LCD panel is stored and processed in the computer system in digital. To drive the transistors in a matrix that correspond to the screen pixels of a TFT LCD, the digital display information must be converted into analog. The audio digital-to-analog converters use a sigma-delta converter for realizing high resolution of greater than 16 bits, and the video DACs for digital TVs, video conference systems, and medical video signal processing systems use a current-matrix converter for high-speed resolution. DACs of the type commonly used for driving video and graphics display devices are called current mode DACs. The currents are steered to either the output or to another node, typically ground, depending on the digital input code presented to the DAC for conversion. As the demands for video signal processing, digital signal synthesis, and high-accuracy digital-to-analog converters (DACs) generate an increased need for higher clock frequencies and dynamic ranges, CMOS current-steering DAC architectures provide the best structure for most of these applications. CMOS current-steering DACS may be integrated within digital circuits to yield systems that have cost and power consumption advantages. CMOS current-steering digital/analog converters are intrinsically faster and more linear than competing architectures such as resistor-string digital/analog converters.
Characteristics that determine the performance quality of a DAC include resolution, sampling rate, linearity and monotonicity. Resolution is the number of bits of digital input code used to produce the corresponding analog output signal. The sampling rate indicates the rate at which the digital input codes are converted to analog outputs. Linearity reflects that for each change in the digital input code, there is a proportionate change in the analog output. The accuracy of the DAC's measurement and conversion is typically specified by the converter's linearity. Integral linearity is a measure of linearity over the entire conversion range. It is defined as the deviation from a straight line drawn between the maximum point and through zero (or the offset value) of the conversion range. Differential linearity is the linearity between adjacent steps of the analog output. Differential non-ninearity is the deviation from ideal linearity of the analog output signal measured between two successive digital input codes. Differential linearity is a measure of the monotonicity of the converter. A DAC exhibits monotonicity if in its transfer characteristic (a graph of the analog output as a function of the digital input), increasing digital input results in increasing analog output, i.e., as the values of the digital input codes increase, the values of the analog outputs never decrease. Thus, a DAC exhibits nonmonotonicity if an incremental increase in the digital input results in an incremental decrease in the analog output. As the use of DACs becomes more prevalent in higher-frequency applications, the DACs dynamic linearity can become a limiting issue. Dynamic linearity refers to the DAC's ability to accurately reproduce higher-frequency analog signals. High dynamic linearity indicates that the DAC will accurately reproduce a tone; whereas, poor dynamic linearity indicates that the DAC will produce unwanted spectral components. Achieving a high dynamic linearity is particularly important in many applications, such as broadband communications. Static linearity refers to the ability of a DAC to reproduce an, accurate analog voltage or current level in response to a received digital word. Improved static linearity can be obtained by providing a cascode device within a current leg, such as a cascode current source, or similar compensating element. Generally, current-mode DACs provide a high degree of linearity at low frequencies but linearity drops off progressively as operational frequency increase towards the Nyquist frequency.
An important quality feature of a digital/analog converter (DAC) is its resolution. Sigma-delta modulation (delta-sigma modulation) provides a high resolution digital-to-analog conversion solution. Sigma-delta DACs have come into widespread use with the development of signal processing and digital audio technologies and their applications. A delta-sigma DAC has a digital input summer, a digital interpolation filter, a digital feedback loop, a quantizer, and a DAC output stage at the modulator output. Sigma-delta DACs commonly include a front-end interpolator which receives digital input samples and increases the sampling rate of the digital input samples. The sigma-delta modulator receives the higher frequency input samples from the interpolator and converts the samples to a lower resolution (typical one-bit), high frequency bit stream. Rather than spreading quantization noise uniformly over the frequency range from 0 to the sampling Nyquist frequency, the sigma delta modulator shapes the noise so that the majority of the noise falls into the very high frequencies above the Nyquist frequency. The performance of a digital-to-analog converter in audio equipment is principally represented by factors such as a distortion factor (a ratio of a harmonic component to a signal) and a signal to noise (SIN) ratio. Sigma-delta modulators, which can be used in sigma-delta converters, can provide a degree of shaping (filtering) of quantization noise that can be present. The higher the order of the sigma-delta modulator, the further the quantization noise is pushed into the frequency band and the greater the separation between the signal being converted and the quantization noise. Delta-sigma modulators are particularly useful in digital-to-analog converter (DAC) systems. Using oversampling, a delta-sigma modulator spreads the quantization noise power across the oversampling frequency band, which is typically much greater than the input signal bandwidth.