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Definition
Analog to digital converter is a device that converts continuously varying analog signals to discrete digital numbers. In other words, analog-to-digital converter samples analog signals to produce a digital signal that describes the original analog signal.
Structure
Flash and SAR circuit design:
The two common architectures used for analog-to-digital converter chips are successive approximations register (SAR) and flash. The difference between SAR circuit design and flash circuit design is that SAR uses a single comparator and multiple conversion cycles while flash (or parallel) architecture uses multiple comparators and a single conversion cycle.
Advantages of flash over SAR
Since flash ADCs use a set of 2n-1 comparators to measure an analog signal to a resolution of n bits, flash ADCs are faster than SAR ADCs.
Downside
Flash require a greater number of comparators
Pipeline structure
The best way of solving a problem is by dividing or segregating the problem into parts and solving it. This exactly how pipeline architecture overcomes some of the limitations carried by flash architecture. Pipeline circuit design converts analog signal to digital signal by dividing the conversion task into several consecutive stages. Each stage of pipeline structure consists of a sample and hold circuit, i.e. a x-bit ADC (e.g., a flash converter), and a x-bit digital-to-analog converter (DAC).
Advantage
With same number of comparators as flash converter, pipelined converters are able to achieve higher resolutions.
Drawback
Although pipelined converters are able to achieve higher resolutions with same number of comparators as flash, pipeline analog-to-digital converter chips requires extra total conversion time from one cycle to n cycles.
Specifications of ADC
The resolution of the converter is the ability of the converter to produce number of discrete values for a range of supplied voltage values, usually expressed in bits (if it has to be electrically defined, it is done in volts). For example, ADC: an ADC with a resolution of eight bits is capable of dividing a supplied analog input in to one of 256 discrete values (0..255)
i.e. 28 = 256.
For instance,
1) Full scale measurement range = 0 to 15 volts
2) ADC resolution is 12 bits: 212 = 4096 quantization levels
Therefore,
ADC voltage resolution is: (15-0)/4096 = 0.00375 volts = 3.75 mV
If the voltage range varies from -15 to +15 volts, then
- Full scale measurement range = -15 to +15 volts
- ADC resolution is 14 bits: 214 = 16384 quantization levels
- ADC voltage resolution is: (15-(-15))/16384 = 30/16384 = 0.00183 volts = 1.83 mV
Note:
Since noise is present in the supplied input, resolution of the converter gets affected, resulting in inaccurate output, as lower bits are in form of noise.
Response type
Response type of ADC depends upon response of output to the input. The response of output to the input is either linear or non-linear.
Linear ADCs
Most ADCs belong to linear type, i.e. there is a linear relationship of input values to the output values, although analog-to-digital conversion is an inherently non-linear process. To be precise, mathematically, the mapping of a continuous space (analog voltage) to a discrete space(digitized voltage) is a non-invertible and therefore non-linear operation.
Non-linear ADCs
The non-linear ADCs are designed in such a way that the nonlinearity of the analog to Digital output carries out a predetermined correction of the signal. The signal-to-noise ratio relative to the quantization noise can be considered best when the probability density function of a signal being digitized is uniform.
Accuracy
Accuracy of ADC indicates extend of accuracy in the conversion of analog signal to digital signal. Errors are introduced in the A to D conversion due quantization error and non-linearity.
Sampling frequency
The frequency at which the new digital values are sampled from the analog signal is called sampling frequency of an analog to digital converter.
Bandwidth
Bandwidth of an analog to digital converter refers to difference between the higher and lower limits of the frequency response.
Applications
The ability of ADC to convert analog signals to digital signal makes it an essential device in electronic industry.
- Music industry: An ADC is used to generate the PCM data stream that is connected onto a compact disc in an analog recording process used for analog recording.
- Analog to digital converter is used in TV tuner cards.
- Microcontrollers use on-board 8, 10, 12 and 16 bit ADCs.
- In devices like digital radio, digital oscilloscopes use very fast analog to digital converters.
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