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An analog to digital converter (abbreviated ADC, A/D or A to D) is an electronic circuit that converts analog voltages to digital number. The basic principle of working of A/D converter is to use the comparator principle to determine whether or not to turn on a particular bit of the binary number output. It is typical for an ADC to use a digital-to-analog converter (DAC) to determine one of the inputs to the comparator.
Types of ADCs
Using different interfaces, ADCs come in various speeds, providing differing degrees of accuracy. The most common types of ADCs are flash, successive approximation, and sigma-delta.
Resolution
The resolution of the converter indicates the number of discrete voltage, state or value the converter can produce over the range of voltage values, usually expressed in bits. For example, an ADC that encodes an analog input to one of 256 discrete values (0 to 255) has a resolution of eight bits.
Therefore, the voltage resolution of an ADC is equal to its overall voltage measurement range divided by the number of discrete values.
for example
o Full scale measurement range = 0 to 10 volts
o ADC resolution is 12 bits: 212 = 4096 quantization levels
o ADC voltage resolution is: (10-0)/4096 = 0.00244 volts = 2.44 mV
In signal processing, oversampling is the process of sampling a signal with a sampling frequency significantly higher than twice the bandwidth or highest frequency of the signal being sampled. An oversampled signal is said to be oversampled by a factor of ß, defined as
ß = fs/ 2 fh
or
fs = 2 ß fh
where,
fs is the sampling frequency
fh is the bandwidth or highest frequency of the signal
Flash ADC
The flash ADC is the fastest type of A/D converter. A flash ADC comprises of comparators, one per voltage step, and a string of resistors. For example, a 4-bit ADC will have 16 comparators, an 8-bit ADC will have 256 comparators. All of the comparator outputs are connected to a block of logic that determines the output depending on high and low state of comparators.
The conversion speed of the flash ADC is the sum of the comparator delays and the logic delay (the logic delay is usually negligible).
Advantages
Flash ADC is the fastest among all comparators.
Downside
Although Flash ADCs are very fast in conversion, they consume enormous amounts of IC real estate. In addition, since the more number of comparators are required, they tend to be power hogs, drawing significant current. E.g., A 10-bit flash ADC may consume half an amp.
Alternative
A variation on the flash converter is the half-flash, which uses an internal digital-to-analog converter (DAC) and subtraction to reduce the number of internal comparators. Half-flash converters are slower than true flash converters but faster than other types of ADCs.
Successive approximation converter
Using a comparator and counting logic, a successive approximation converter performs a conversion.
Working
These type of converters starts conversion by comparing input voltage with the reference voltage. If the input is greater than half the reference voltage, the most significant bit (MSB) of the output is set. This value is then subtracted from the input, and the result is checked for one quarter of the reference voltage. This process continues until all the output bits have been set or reset. A successive approximation ADC takes as many clock cycles as there are output bits to perform a conversion.
Sigma-delta
In order to obtain accurate conversion, A sigma-delta ADC uses a 1-bit DAC and performs filtering, and oversampling. The input reference and the input clock rate control the conversion accuracy.
Advantages
-High resolution can be obtained by using sigma-delta converter.
-Since the flash and successive approximation ADCs use a resistor ladder or resistor string, the accuracy of the resistors directly affects the accuracy of the conversion result. Although modern ADCs use very precise, laser-trimmed resistor networks, some inaccuracies still persist in the resistor ladders. The sigma-delta converter does not have a resistor ladder but instead takes a number of samples to converge on a result.
Disadvantages
Since Sigma converter works by oversampling the input, the conversion takes many clock cycles. Hence, the speed of the sigma-delta converter is affected. Thus, for a given clock rate, the sigma-delta converter is slower than other converter types. In other words, for a given conversion rate, the sigma-delta converter requires a faster clock.
Complexity of the digital filter that converts the duty cycle information to a digital output word is another big disadvantage of the sigma-delta converter. The sigma-delta converter has become more commonly available with the ability to add a digital filter or DSP to the IC die.
Applications
- TV tuner cards (to convert full-speed analog video to MPEG digital video file)
- Microcontrollers
- Digital oscilloscopes
- C.P.U
- Video cameras, and in all devices where analog signal has to be converted into digital signal.
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