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Compare and explain different types of ADCs.
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An Analog-Digital Converter (ADC) is a widely used electronic component that converts an analog electric signal (usually a voltage) into a digital representation. The ADCs are at the front-end of any digital circuit that needs to process signals coming from the exterior world. Its schematic symbol is: The output of a microphone, the voltage at a photodiode, or the signal of an accelerometer are examples of analog values that need to be converted so that a microprocessor can work with them.

All ADCs work under the same principle: they need to convert a signal to a certain number of bits N. The sequence of bits represents the number and each bit has the double of the weight of the next, starting from the Most Significant Bit (MSB) up to the Least Significant Bit (LSB). In a nutshell, we want to find the sequence of bits $b_{(N-1)}, b_{(N-2)},..., b_0$, that represents the analog value $V_{in}$ as

$V_{in}=\sum_{n=0}^{N-1}b_n2^n\frac{V_{ref}}{2^N}$

The MSB has weight $\frac{V_{ref}}2$, the next $\frac{V_{ref}}4$, etc., and the LSB has weight $\frac{V_{ref}}{2N}$. Therefore, more bits lead to more precision in the digital representation. Here we simplify the range to be between 0 and $V_{ref}$, although the range may be between any two values.

Following are few types of ADCs:

• Flash

• Successive-Approximations Register (SAR)

• Integrating or Dual-slope

Flash ADCs -

Flash converters have a resistive ladder that divides the reference voltage in 2N equal parts. For each part, a comparator compares the input signal with the voltage supplied by that part of the resistive ladder. The output of all the comparators is like a thermometer: the higher the input value, more comparators have their outputs high from bottom to top. A dedicated component called "Priority Encoder" translates this gauge into a binary code, which corresponds to the position of the last comparator with high output, counting from the bottom up.

Strengths –

• Very fast, converts instantly

Weaknesses -

• It doubles in size for each bit added to the representation. N bits require 2N−1 comparators

• It has a high input capacitance (the input capacitance of a comparator multiplied by 2N−1)

• It consumes a lot of power

Successive Approximation Register (SAR) ADCs -

A Successive Approximation Register converter evaluates each bit at a time, from the most to the least significant bits. They successively approach the output of a digital-analog converter (DAC) in them to the input voltage. The input of the DAC is stored in a N bit register, which is also the output of the ADC.

Strengths

• It uses only one comparator

• Low power consumption

Weaknesses -

• The DAC grows with the number of bits

• They take as many cycles to convert the signal as the number of bits

• The component mismatch in the DAC limits its linearity (and therefore of the ADC) to around 12bits

Integrating or Dual-slope DACs -

The dual-slope are very precise, but slow converters that use counters to generate the output. As its name suggests, this converter has 2 phases, the first where a voltage ramps up with a certain slope, and the second where the same voltage ramps down with a different slope.

Strengths -

• Very precise. The sources of errors are only the comparison with zero and the clock period Weaknesses -

• Slow. The ADC needs time to ramp up and down the output voltage and doubles with each bit added to the representation, for a fixed clock period In addition to the above mentioned three ADCs, there are also other types like Pipelined converters and Sigma Delta converters.

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