Digital Electronics - Data Converters Complete Study Notes

By Gourabh Kumar|Updated : February 7th, 2022

Complete coverage of the UPPCL AE Exam syllabus is a very important aspect for any competitive examination but before that important subjects and their concept must be covered thoroughly. In this article, we are going to discuss the fundamental of Data Converters which is very useful for UPPCL AE Exams.

DAC and ADC: It is possible to convert the analog signal to digital and vice-versa. We can get analog from digital through DAC and can get digital from analog through ADC.

Digital to Analog Converter (DAC): D/A converter (also called a DAC) accepts an n-bit digital word and produces an analog sample.

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where, k = Proportionality factor, bn = 1;if nth bit of digital input is 1, bn = 0; if nth bit of digital input is 0.

Classification of DAC:

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Weighted Resistor DAC (N – bit):

  • A DAC can be constructed by using a Summing Amplifier and a set of resistors R, 2R, 4R, 8R, etc as its inputs.
  • The circuit consists of a reference voltage Vf, N binary-weighted resistors R, 2R, 4R, 8R, . . . , 2(N-1) R, N single-pole double-throw switches, and an Op-amp together with its feedback resistance R = R/2.
  • The switches are controlled by an N-bit digital input word D.

1

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V0 = -Rf If = -Vf D

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LSB resistance = (2N – 1) MSB resistance.

  • The accuracy of the DAC depends critically on the accuracy of the Reference voltage, the precision of the binary-weighted resistors, and the perfection of the switches.
  • A disadvantage of the binary-weighted resistor network is that for a large number of bits (N > 4) the spread between the smallest and largest resistances becomes quite large. This implies difficulties in maintaining accuracy in resistor values.

R – 2R Ladder DAC: Non-Inverting OP-amp type DAC

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Inverting Amplifier

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Inverted Ladder R – 2R Circuit

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Specifications for DAC:

  • Resolution in DAC is changed in analog output with corresponding to 1 LSB bit increment at the input.
  • Resolution = weight of LSB =
  • V= Voltage corresponding to logic N = Number of bits.
  • Analog Output Analog output = resolution x decimal equivalent of binary data
  • Maximum Analog Output Voltage (VFS) VFS is the maximum analog output voltage of DAC.

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VFS = Vr

  • Percentage Resolution:

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  • Maximum Error Maximum error acceptable in ADC and DAC equals to resolution.
  • Resolution (R-2R ladder type) = image020

Analog to Digital Converter:

  • A/D converter (also called an ADC) accepts an analog sample Vand produces an N-bit digital word.
  • Examples of ADC usage are digital volt meters, cell phone, thermocouples, and a digital oscilloscope.
  • Types of A/D Converters: Dual Slope A/D Converter, Successive Approximation A/D Converter, Flash A/D Converter, Delta-Sigma A/D Converter, etc.

Counter type ADC:

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  • In N-bit counter type ADC:
    • Maximum number of clock pulses required for conversion = 2N -1
    • Maximum time required for conversion = (2N -1) TCLK
    • Minimum number of clock pulses = 1
    • Average number of clock pulses = 2N-1

Successive Approximation Type ADC:

  • It is faster than digital ramp ADC.
  • Conversion time (tc) is independent of the value of the analog input voltage (Va).
  • It has fixed conversion time.

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  • Maximum number of clock pulses = N for conversion
  • Maximum conversion time = N.TCLK

Flash Type ADC:

  • It is also known as Parallel-comparator type ADC or Simultaneous converter.
  • It is highest speed ADC (fastest ADC)
  • Functional component
  • It utilizes 2N -1 comparators to compare the input signal level with each of the 2N -1 possible quantization levels.
  • The outputs of the comparators are processed by an encoding-logic block to provide the N bits of the output digital word.
  • Complete conversion can be obtained within one clock cycle.
  • For N-bit comparator:
    • Total number of comparators = 2N -1,
    • Total number of resistors = 2N,
    • Total number of priority encoders = 1 (2N × N)

2-bit Flash Converter:

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Dual Slope Integrating Type ADC:

  • It has slowest conversion time but has relatively low cost.
  • The following components are present in the Dual slope A/D converter:
    • Integrator
    • Electronically Controlled Switches
    • Counter
    • Clock
    • Control Logic
    • Comparator

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where, n = Count recorded in the counter.

Dual Slope Integrating Type ADC:

  • Total number of clock pulses = 2N + n
  • Maximum number of clock pulses = 2N + 2N - 1 = 2N + 1 -1== 2N + 1

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