Mumbai University > Computer Engineering > Sem 3 > Electronic Circuits and Communication Fundamentals

Marks: 10 Marks

Year: May 2014

Following are few types of ADCs:

1. Flash
2. Successive-Approximations Register (SAR)
3. 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 andSigma Delta converters.

Binary weighted DAC

A binary-weignted resistor DAC for N-bit has N resistors which are binary weighted $R, 2R,….2^{N-1}R$, a reference voltage $V_{ref}$ is connected, and a feedback resistance R_f=R/2. The circuit also has N switches.

$$\text{Digital input word}=\dfrac{b_1}{2}+\dfrac{b_2}{2^2}+\dfrac{b_3}{2^3}+....+\dfrac{b_N}{2^N}$$

Control the switches where $b_1, b_2$ are the coefficients having binary values, $b_1$ represents the MSB and $b_N$ represents the LSB. Switch $S_1$ is controlled by $b_1$ and $S_2$ by $b_2$ and so on. When the bit (b) is zero, the switch moves to position 1 and when the bit (b) is one, the switch moves to position 2.

Position 1 is the ground for all switches. When the switches are in position , the currents add up and the total current flows through feedback resistor, the total current is given as,

$$i=\dfrac{V_{ref}}{R}.b_1+\dfrac{V_{ref}}{2R}.b_2+....+\dfrac{V_{ref}}{2^{N-1}.R}b_N \\ i==\dfrac{2V_{ref}}{R}\bigg(\dfrac{b_1}{2}+\dfrac{b_2}{2^2}+....+\dfrac{b_N}{2^N}\bigg)$$

And the output voltage as,

$$V_{out}=-i.R_f=\dfrac{-2V_{ref}}{R}\times\dfrac{R}{2}\bigg[\dfrac{b_1}{2}+\dfrac{b_2}{2^2}+....+\dfrac{b_N}{2^N}\bigg] \\ V_{out}=-V_{ref}\bigg[\dfrac{b_1}{2}+\dfrac{b_2}{2^2}+....+\dfrac{b_N}{2^N}\bigg]$$

Thus, $V_{out}$ is proportional to the digital input word

$$\bigg[\dfrac{b_1}{2}+\dfrac{b_2}{2^2}+....+\dfrac{b_N}{2^N}\bigg]$$