This chapter describes the functioning of an analog to digital converter using PIC18F Analog to Digital Converters Analog signals are continuous time-varying signals. Most of the real world signals around us are analog in nature. Analog to digital conversion becomes inevitable when these real-world signals need to be processed in a digital environment.
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Overview[ edit ] Ideally sampled signal. Piecewise constant output of a conventional DAC lacking a reconstruction filter. In a practical DAC, a filter or the finite bandwidth of the device smooths out the step response into a continuous curve. A DAC converts an abstract finite-precision number usually a fixed-point binary number into a physical quantity e. In particular, DACs are often used to convert finite-precision time series data to a continually varying physical signal.
An ideal DAC converts the abstract numbers into a conceptual sequence of impulses that are then processed by a reconstruction filter using some form of interpolation to fill in data between the impulses. A conventional practical DAC converts the numbers into a piecewise constant function made up of a sequence of rectangular functions that is modeled with the zero-order hold.
Other DAC methods such as those based on delta-sigma modulation produce a pulse-density modulated output that can be similarly filtered to produce a smoothly varying signal.
As per the Nyquist—Shannon sampling theorem , a DAC can reconstruct the original signal from the sampled data provided that its bandwidth meets certain requirements e. Digital sampling introduces quantization error that manifests as low-level noise in the reconstructed signal.
To illustrate, consider a typical long-distance telephone call. The digital stream is then divided into network packets where it may be sent along with other digital data , not necessarily audio. The packets are then received at the destination, but each packet may take a completely different route and may not even arrive at the destination in the correct time order.
The digital voice data is then extracted from the packets and assembled into a digital data stream. A DAC converts this back into an analog electrical signal, which drives an audio amplifier , which in turn drives a loudspeaker , which finally produces sound.
Audio[ edit ] Top-loading CD player and external digital-to-analog converter. Most modern audio signals are stored in digital form for example MP3s and CDs and, in order to be heard through speakers, they must be converted into an analog signal. Specialist standalone DACs can also be found in high-end hi-fi systems. These normally take the digital output of a compatible CD player or dedicated transport which is basically a CD player with no internal DAC and convert the signal into an analog line-level output that can then be fed into an amplifier to drive speakers.
Similar digital-to-analog converters can be found in digital speakers such as USB speakers, and in sound cards. Given this inherent distortion, it is not unusual for a television or video projector to truthfully claim a linear contrast ratio difference between darkest and brightest output levels of or greater, equivalent to 10 bits of audio precision even though it may only accept signals with 8-bit precision and use an LCD panel that only represents 6 or 7 bits per channel.
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. A device that is distantly related to the DAC is the digitally controlled potentiometer , used to control an analog signal digitally.
Play media IBM Selectric typewriter uses a mechanical digital-to-analog converter to control its typeball. A one-bit mechanical actuator assumes two positions: one when on, another when off. The motion of several one-bit actuators can be combined and weighted with a whiffletree mechanism to produce finer steps.
The IBM Selectric typewriter uses such a system. High-speed DACs are used for mobile communications and ultra-high-speed DACs are employed in optical communications systems. Types[ edit ] The most common types of electronic DACs are:  The pulse-width modulator where 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 dimming LED lamps. Oversampling DACs or interpolating DACs such as those employing delta-sigma modulation , use a pulse density conversion technique with oversampling. The binary-weighted DAC, which contains individual electrical components for each bit of the DAC connected to a summing point, typically an operational amplifier.
Each input in the summing has powers-of-two values with most current or voltage at the most-significant bit. 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. Individual resistors are enabled or bypassed in the network based on the digital input. Switched current source DAC, from which different current sources are selected based on the digital input.
Switched capacitor DAC contains a parallel capacitor network. Individual capacitors are connected or disconnected with switches based on the input.
This improves the precision due to the relative ease of producing equal valued-matched resistors. The successive approximation or cyclic DAC,  which successively constructs the output during each cycle. Individual bits of the digital input are processed each cycle until the entire input is accounted for. The thermometer-coded DAC, which contains an equal resistor or current-source segment for each possible value of DAC output.
This is a fast and highest precision DAC architecture but at the expense of requiring many components which, for practical implementations, fabrication requires high-density IC processes. Most DAC integrated circuits are of this type due to the difficulty of getting low cost, high speed and high precision in one device.
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 by the use of the thermometer-coded principle and number of resistors or current sources by the use of the binary-weighted principle.
Most DACs shown in this list rely on a constant reference voltage or current to create their output value. Alternatively, a multiplying DAC  takes a variable input voltage or current as a conversion reference. This puts additional design constraints on the bandwidth of the conversion circuit.
Their output signals are combined in the analog domain to enhance the performance of the combined DAC. This is usually stated as the number of bits it uses, which is the binary logarithm of the number of levels.
Resolution is related to the effective number of bits which is a measurement of the actual resolution attained by the DAC. Resolution determines color depth in video applications and audio bit depth in audio applications.
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