In electronics, an analog-to-digital converter (abbreviated ADC, A/D, or A to D) is a device that converts continuous signals to discrete digital numbers.
Typically, an ADC converts a voltage to a digital number. A digital-to-analog converter (DAC) performs the reverse operation.
Resolution
The resolution of the converter indicates the number of discrete values it can produce. It is usually analog video to digital video converter expressed in bits. For example, an ADC that encodes an analog input to one of 256 discrete values has a resolution of eight bits, since
- 28 = 256.
Resolution can also be defined electrically, and expressed in volts. The voltage resolution of an ADC is equal to its overall voltage measurement range divided by the number of discrete values. Some examples may help:
- Example 1
- Full scale measurement range = 0 to 10 volts
- ADC resolution is 12 bits: 212 = 4096 quantization levels
- ADC voltage resolution is: (10-0)/4096 = 0.00244 volts = 2.44 mV
- Full scale measurement range = 0 to 10 volts
- Example 2
- Full scale measurement range = -10 to +10 volts
- ADC resolution is 14 bits: 214 = 16384 quantization levels
- ADC voltage resolution is: (10-(-10))/16384 = 20/16384 = 0.00122 volts = 1.22 mV
- Full scale measurement range = -10 to +10 volts
In practice, the resolution of the converter is limited by the signal-to-noise ratio of the signal in question. If there is too much noise present in the analog input, it will be impossible to accurately resolve beyond a certain number of bits of resolution. While the ADC will produce a result, the result is not accurate, introduction to digital to analog converter buy analog to digital tv converter since its lower bits are simply measuring noise. The S/N ratio should be around 6 dB per bit of resolution required.
Response type
Most ADCs are of a type known as linear, although analog-to-digital conversion is an inherently analog to digital converter non-linear process (since the mapping of a continuous space to a discrete space is a non-invertible and therefore non-linear operation). In the sense of the term "linear" as used here, it means that the range of the input values that map to each output value has a linear relationship with the output value, i.e., that the output value k is used for the range of input values from m*(k+b) to m*(k+1+b) where m and b are some constants and b is typically equal either to 0 or to -0.5. When b = 0, the ADC is referred to as mid-rise, and when b = -0.5 it is referred to as mid-tread.
Human ears can just barely detect changes in sound levels of 1 dB -- a logarithmic unit (see Decibel#Acoustics ). In other words, human hearing is very sensitive at low amplitudes, and less sensitive at high amplitudes.
The logarithmic ADC has the same characteristics. Logarithmic ADCs are very common in voiced communication systems to increase the dynamic range of the representable values while retaining fine-granular fidelity in the low-amplitude region.
An 8 bit a-law basic principle of analog to digital converter or the μ-law logarithmic ADC covers the wide dynamic range and has the high resolution (at least in the critical low-amplitude region) that would otherwise require a 12 bit linear ADC.
Accuracy
Accuracy depends on the error in the conversion. If the ADC is not broken, this error has two components: quantization error and (assuming the ADC is intended to be linear) non-linearity. These errors are measured in a unit called the LSB, which is an abbreviation for least significant bit. In the above example of an eight-bit ADC, an error of one LSB is digital to analog converter card 1/256 of the full signal range, or about 0.4%.
Quantization error is due to the finite resolution of the ADC, and is an unavoidable imperfection in all types of ADC. The magnitude of the quantization error at the sampling instant is between zero and half of one LSB.
In the general case, the sampled signal is larger than one LSB, and the quantization error is not correlated with the signal. Its RMS value is then . In the eight-bit ADC example, this represents analog to digital converter cable 0.113 % of the full signal range.
All ADCs ratiometric analog to digital converter suffer from non-linearity errors caused by their physical imperfections, causing their output to deviate from a linear function (or some other function, in the case of a deliberately non-linear ADC) of their input. These errors can sometimes be mitigated by calibration, or prevented by testing.
Important parameters toslink digital to rca analog converter for linearity are integral non-linearity (INL) and differential non-linearity (DNL).
Sampling rate
The analog signal is continuous in time and it is necessary to convert this to a flow of digital values. It is therefore required to define the rate at which new digital values are sampled from the analog signal. The rate of new values is called the sampling rate or sampling frequency of the converter.
A continuously varying bandlimited signal can be sampled (that is, the signal values at intervals of time T, the sampling time, are measured and stored) and then the original signal can be exactly reproduced from the discrete-time values by an interpolation formula. The accuracy is however limited by quantization error. However, this faithful reproduction is only possible if the sampling rate is higher than twice the bandwidth of the signal. digital to analog converter chip This is essentially what is embodied in the Shannon-Nyquist sampling theorem.
Since a practical ADC cannot make an instantaneous conversion, the input value must necessarily be held constant during the time that the converter performs a conversion (called the conversion time). An input circuit called a sample and hold performs this task—in most cases by using a capacitor to store the analogue voltage at the input, and using an electronic switch or gate to disconnect the capacitor from the input. Many ADC integrated circuits include the sample and hold subsystem internally.
Aliasing
All ADCs work by sampling their input at discrete digital to analog converter intervals of time. Their output is therefore an incomplete picture of the behaviour of the input. There is no way of knowing, analog to digital aes converter media converter analog to digital by looking at the output, what the input was doing between one sampling instant and the next. If the input is known to be changing slowly compared to the sampling rate, then it can be assumed that the value of the signal between two sample instants was somewhere between the two sampled values. If, however, the input signal is changing fast compared to the sample rate, then this assumption is not valid.
If the analog to digital video converter digital values produced by the ADC are, at some later stage in the system, converted back to analog values by a digital to analog converter or DAC, it is desirable that the output of the DAC be a faithful representation of the original signal. If the input signal is changing much faster than the sample rate, then this analog to digital converter 8 mm will not be the case, and spurious signals called aliases will be produced at the output of the DAC. The audio digital to analog converter frequency of the aliased signal is the difference between the signal frequency and the sampling rate. For example, a 2 kHz sinewave being sampled at 1.5 kHz would be reconstructed as a 500 Hz sinewave. This problem is called aliasing.
To avoid aliasing, the input to an ADC must be low-pass filtered to remove frequencies above half the sampling rate. This filter is called mcintosh mx132 digital to analog converter an anti-aliasing filter, and is essential for a practical ADC system.
Dither
In A to D converters, performance can be improved using dither. This is a very how to build an analog to digital telephone converter small amount of random noise (white noise) which is added to the input before conversion. Its amplitude is set to be about half of the least significant bit. Its effect is to cause the state of the LSB to randomly oscillate analog to digital converter space applications mass between 0 and 1 in the presence of very low levels of input, music converter from analog to digital rather than digital tv to analog tv converter sticking at a fixed value. Rather than the signal simply getting cut off altogether at this low level (which is only being quantized to a resolution of 1 bit), it extends the effective range of signals that the A to D converter can convert, at the expense of a slight increase in noise - effectively the quantization error is diffused across a series of noise values which is far less objectionable than a hard cutoff. The result is an accurate representation of the signal over time. A suitable filter at the output of the system can thus recover this small signal variation.
Undithered digital audio sounds extremely distorted and unpleasant. Without dither the low level always digital phone to analog phone converter yields a '1' from the A to D. With dithering, the true level of the audio is still recorded as a series of values over time, digital to analog converter applications rather than a series of separate bits at one instant in time.
A virtually identical process, also called dither or dithering, is often used when quantizing photographic images to a fewer number of bits per pixel - the image becomes noisier but to the eye looks far more realistic than the quantized image, which otherwise becomes banded. This analogous process may help to visualize the effect of dither on an analogue audio signal that is converted to digital.
Dithering is also used in integrating systems such as electricity meters. Since the values are added together, the dithering produces results that are more exact than the LSB of the analog-to-digital converter.
ADC structures
There are five common ways of implementing an electronic ADC:
- A direct conversion ADC or flash ADC has a comparator that fires for each a/v analog to digital converter decoded voltage range. The comparator bank feeds a logic circuit that generates a code for each voltage range. Direct conversion is very fast, but usually has only 8 bits of resolution (256 comparators) or less, as it needs a large, expensive circuit. ADCs of this xitel inport analog to digital converter box hfl-i1-x1 type have a large die size, a high input capacitance, and are prone to produce glitches on the output (by outputting an out-of-sequence code). They are often used for video or other fast signals.
- A successive-approximation ADC uses a comparator to reject ranges of voltages, analog to digital monitor converter eventually settling on a final voltage range. For example, the first comparison might decide the most significant bit of the output, the next comparison decides the next-most significant bit, etc. This is also digital to analog video converter called bit-weighting conversion, and is similar to a binary search. ADCs of this type convert very fast, and have good resolutions and quite wide ranges. They digital to analog audio converter are more complex than some other designs.
- A delta-encoded ADC has an up-down counter that feeds a digital to analog converter (DAC). The input signal and the DAC both go to a comparator. The comparator controls the counter. The circuit uses negative feedback from the comparator to adjust the counter digital to analog converter audio until the DAC's output is close enough to the input signal. The number is read from the counter. Delta converters have very wide ranges, and high resolution, but the conversion time is dependent on the input signal level, though it will always have a guaranteed worst-case. Delta converters are often very good choices to read real-world signals. Most signals from physical systems do not change abruptly. Some converters combine the delta and successive approximation approaches; this works especially well when high frequencies are known to be small in magnitude. analog to digital converter hysteresis
- A ramp-compare ADC (also called integrating, dual-slope or multi-slope ADC) produces a saw-tooth signal that ramps up, then quickly falls to zero. When the ramp starts, a timer starts counting. When the ramp voltage matches digital converter analog to digital stereo audio the input, a comparator fires, and the timer's dac08 digital to analog converter value is recorded. Timed ramp converters require the least number of transistors. The ramp time is sensitive to temperature because the circuit generating the ramp is often just some building a digital to analog converter dac simple oscillator. There are two solutions: use a clocked counter driving a DAC and then use the comparator to preserve the counter's value, or calibrate the timed ramp. A special advantage of the ramp-compare system is that comparing a second signal just requires another comparator, and another register to store the voltage value.
- A pipeline ADC (also called subranging quantizer) uses two or more steps of subranging. First, a coarse conversion is done. In a second step, the difference to the input signal is determined with a digital to analog converter digital to analog find analog to digital signal converter converter al 15 (DAC). This difference is then converted finer, and the results are combined in a last step. This type of ADC is fast, has a high resolution and only requires a small die size.
- A Sigma-Delta ADC (also known as a Delta-Sigma ADC) oversamples the desired signal by a large factor and filters the desired signal band. Generally a smaller number of bits than required are converted using a Flash ADC after the Filter. build a digital to analog audio converter The resulting signal, along with the error generated by the discrete levels of the Flash, is fed back and subtracted from the input to the filter. This negative feedback has the effect of noise shaping the error due to the Flash so that it does not appear in the desired signal frequencies. A digital filter (decimation filter) follows the ADC which reduces the sampling rate, filters off unwanted noise signal and increases video analog to digital converter the resolution of the output. (sigma-delta modulation, also called delta-sigma circuit of analog to digital converter modulation)
Nonelectronic multi channel analog to digital converter ADCs usually use some scheme similar to one of the above.
Commercial analog-to-digital converters
These are usually integrated circuits.
Most analog to digital audio converter converters sample with 6 to 24 bits of resolution, and telephone digital to analog converter produce fewer than 1 megasample per second. Mega- and gigasample converters are available, though (Feb 2002). Megasample converters are required in digital video cameras, video capture cards, and TV tuner cards to convert full-speed analog digital to analog converter operational amplifier video to MPEG digital video files. Commercial converters usually have ±0.5 to ±1.5 LSB error in their output.
The most expensive part of an integrated circuit is the pins, because that makes the package larger, and each pin has to be connected to the integrated circuit's silicon. To save pins, it's common for ADCs to send their data one bit at a time over a serial interface to the computer, with the next bit coming out when a clock signal changes state, say from zero to 5V. This saves quite a few pins on the ADC package, and in many cases, does not make the overall design any more complex. (Even microprocessors which use memory-mapped IO only need a few bits of a port to implement a serial bus to an ADC.)
Commercial ADCs often have several inputs that feed the same converter, usually through an analog multiplexer. Different models of ADC may include sample and hold circuits, instrumentation amplifiers or differential inputs, where the quantity measured is the difference between two voltages.
Application to music recording
ADCs are integral to much current music reproduction technology, since much music production is done on analog to digital aes/ebu converter computers; even when analog recording is used, an ADC is still needed to create the PCM high speed analog to digital converter data stream that goes onto a compact disc.
The current crop of AD converters utilized in music can sample at rates up to 192 kilohertz. Many people in the business consider this an overkill and pure marketing hype, due to the Nyquist-Shannon sampling theorem. Simply put, they say the analog waveform does not have enough information in it to necessitate such high sampling rates, and typical recording techniques for high-fidelity audio are usually sampled at either 44.1 KHz (the standard for CD) or 48 KHz (more typical for computer use). However, this kind of bandwidth headroom allows the use of cheaper or faster anti-aliasing filters.
AD converters for musical purposes come in a variety of price ranges - from under $100 to over $10,000 for 2 channels or inputs.
Other applications
AD converters are used virtually everywhere where an analog signal has to be processed, stored, or transported in analog to digital converter board digital form. Fast video ADCs digital pbx to analog phone converter are used, for example, in TV tuner cards. Slow on-chip 8, 10, 12, or 16 bit ADCs are common in microcontrollers. Very fast ADCs are needed in digital oscilloscopes, and are crucial for new applications like software defined radio.
See also
- Digital signal processing
- Modem
- quantization noise
External links
- Learning by Simulations A simulation showing the effects of sampling frequency and ADC resolution.
Electronics Topics
The field of electronics is the study and use of systems that operate by controlling the flow of electrons or other electrically charged particles in devices such as thermionic valves and semiconductors. The design and construction of electronic circuits to solve practical problems is part of the fields of electronic engineering, and the hardware design side of computer engineering. The study of new semiconductor devices and their technology is sometimes considered as a branch of physics.
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