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ADC

Module-4

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INTRODUCTION

  • Analog quantities such as voltages and currents can be processed by amplifiers, voltage dividers, and filters to produce another analog output
  • Digital circuits process numbers and often analog signals are converted to numerical representations for digital recording, transmission, computation and so on.
  • Once processed these quantities are converted back to analog form

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  • A circuit that converts analog signals to digital representation is called as an analog to digital converter.
  • Conversion of digital signal back to analog form is done using digital to analog converter
  • Many A/D conversion methods involve logic circuits.

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ANALOG TO DIGITAL CONVERSION

+

-

1V

1000 Discrete Levels

1 mV

A number is assigned to 1V------- 1000

Dividing an analog voltage into discrete levels is termed a QUANTIZATION and the error involved is known as QUANTIZATION ERROR.

When 1V is divided into 1000 discrete levels, it is said to have a RESOLUTION of 1 in 1000 or 1mV

 

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ANALOG TO DIGITAL CONVERSION

+

-

1V

100 Discrete Levels

10 mV

A number is assigned to 1V------- 100

Dividing an analog voltage into discrete levels is termed a QUANTIZATION and the error involved is known as QUANTIZATION ERROR.

When 1V is divided into 100 discrete levels, it is said to have a RESOLUTION of 1 in 100 or 10mV

 

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An ADC quantizes and converts it into binary code (consisting of zeroes and ones)

ADC

 

 

 

+

-

Input Voltage

Digital Code

​

Analog Voltage

​

​

​

​

0

0

0

0V

0

0

1

1V

0

1

0

2V

0

1

1

3V

1

0

0

4V

1

0

1

5V

1

1

0

6V

1

1

1

7V

There is no binary code for 8V level

Next higher code after 111 is 000 or 1000

 

 

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A 5- bit ADC has a higher precision than a 3-bit ADC

A 10-bit ADC has a higher precision than a 5-bit ADC

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LSB and MSB

 

 

 

 

 

Digital Code

​

Analog Voltage

​

​

​

​

​

​

0

0

0

0

0

0

0

0

0

0

1

31.25mV

0

0

0

1

0

62.5mV

0

0

0

1

1

93.75mV

0

0

1

0

0

125mV

0

0

1

0

1

156.25mV

0

0

1

1

0

187.5mV

0

0

1

1

1

218.75mV

0

1

0

0

0

250mV

0

1

0

0

1

281.25mV

0

1

0

1

0

312.5mV

0

1

0

1

1

343.75mV

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A 500mV level is to be converted into a 7-bit digital code. Determine the resolution of the conversion, the analog levels represented by the LSB and the MSB, and calculate the analog level represented by 1111111

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DIGITAL TO ANALOG CONVERSION

DAC

 

 

 

 

 

Analog Output

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SETTLING TIME

  •  

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MONITONICITY

  • A digital/analog converter should always produce an increase in output when the input increases and a decreased output when the output decreases.
  • When this is the case, the converter is said to be monotonic.
  • Errors will result if the converter is not monotonic.

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ACCURACY

  • The absolute accuracy of the analog/digital conversion involves linearity of the input/output relationship and possible offset error.
  • Offset error is a constant error that produces an output that is larger or smaller than it should be by an amount of fixed amount at every level.
  • Commercially available converters have relative accuracy specified with a maximum error expressed as a percentage of the maximum output.

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WEIGHTED RESISTOR DAC

+

-

 

 

 

 

 

 

 

 

 

 

 

 

​

​

​

​

Analog

Voltage

0

0

1

0.25mA

0.25mA*4kΩ= 1V

0

1

0

0.5mA

2V

0

1

1

0.25mA+0.5mA=0.75mA

3V

1

0

0

1mA

4V

0

0

5V

5kΩ

10kΩ

20kΩ

0.25mA

4kΩ

 

0.25mA

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WEIGHTED RESISTOR DAC

+

-

 

 

 

 

 

 

 

 

 

 

 

 

​

​

​

​

Analog

Voltage

0

0

1

0.25mA

0.25mA*4kΩ= 1V

0

1

0

0.5mA

2V

0

1

1

0.25mA+0.5mA=0.75mA

3V

1

0

0

1mA

4V

0

5V

0

5kΩ

10kΩ

20kΩ

0.5mA

4kΩ

0.5mA

 

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WEIGHTED RESISTOR DAC

+

-

 

 

 

 

 

 

 

 

 

 

 

 

​

​

​

​

Analog

Voltage

0

0

1

0.25mA

0.25mA*4kΩ= 1V

0

1

0

0.5mA

2V

0

1

1

0.25mA+0.5mA=0.75mA

3V

1

0

0

1mA

4V

0

5V

5V

5kΩ

10kΩ

20kΩ

0.25mA

0.5mA

4kΩ

0.75mA

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WEIGHTED RESISTOR DAC

+

-

 

 

 

 

 

 

 

 

 

 

 

 

​

​

​

​

Analog

Voltage

1

0

0

1mA

5V

1

1

0

1.5mA

6V

1

1

1

1.75mA

7V

5V

0

5kΩ

10kΩ

20kΩ

1mA

0

1mA

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WEIGHTED RESISTOR DAC

+

-

 

 

 

 

 

 

 

 

 

 

 

 

​

​

​

​

Analog

Voltage

1

0

0

1mA

5V

1

1

0

1.5mA

6V

1

1

1

1.75mA

7V

5V

5V

5kΩ

10kΩ

20kΩ

0.25mA

0.5mA

1mA

5V

1.75mA

 

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Advantages of Weighted Resistor DAC

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Disadvantages of DAC

  • The resistor currents in the weighted resistor DAC change according to the presence or absence of each digital input bit. This causes changes in the heat dissipated in each resistor which produces voltage drop errors.
  • Many different values of resistors are required, especially in case of higher bit number converters
  • This makes the weighted resistor DAC inconvenient for integrated circuit fabrication.

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R-2R DAC

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-

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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R-2R DAC

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R-2R DAC

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-

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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R-2R DAC

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-

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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R-2R DAC

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-

 

 

 

 

 

 

 

 

10k Ω

​

​

 

 

 

 

 

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R-2R DAC

+

-

 

 

 

 

 

 

 

​

5 kΩ

 

 

 

 

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R-2R DAC

+

-

 

 

 

 

 

 

​

10 kΩ

 

 

 

 

 

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R-2R DAC

+

-

 

 

 

 

 

 

 

 

 

 

 

 

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R-2R DAC

+

-

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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R-2R DAC

+

-

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

001----🡪 0.25mA*5kΩ =1.25V

=0.25mA

​

 

 

0.25mA

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R-2R DAC

+

-

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

011----🡪 0.75mA*5kΩ =3.75V

=0.75mA

​

 

0.5mA

​

0.25mA

​

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R-2R DAC

+

-

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

111----🡪 1.75mA*5kΩ =8.75V

=1.75mA

​

 

1mA

​

0.5mA

​

0.25mA

​

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If the R-2R DAC circuit is expanded into a 5-bit DAC, calculate the analog output voltage when the inputs are (a) 00001�(b) 10000 (c) 11111

 

 

 

 

 

 

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Integrated Circuit 8-bit D/A

  •  

 

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Linear Ramp ADC

+

-

 

 

 

Ramp Generator

 

Clock Generator

​

​

​

​

​

 

 

 

 

 

Register

H

0

0

1

1

1

Reset

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Linear Ramp ADC

+

-

 

 

 

Ramp Generator

 

Clock Generator

​

​

​

​

​

 

 

 

 

 

Register

L

0

0

1

1

1

No Clock Pulses

0

0

0

Reset=0

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Comparator

output

​

 

 

 

 

 

 

Clock pulses

​

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A linear ramp ADC uses a 10-bit counter and a 15kHz clock frequency. The counter output is to be 1111111111 when the input voltage is 100mV. Calculate the required ramp rate of change and the ADC conversion

  •  

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SUCCESSIVE APPROXIMATION ADC

+

-

 

 

 

DAC

 

Clock Generator

​

​

​

​

​

 

 

 

 

 

Register

H

​

0

0

H

1

0

0

0V

Reset

Control

H

0

0

1

8V

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SUCCESSIVE APPROXIMATION ADC

+

-

 

 

 

DAC

 

Clock Generator

​

​

​

​

​

 

 

 

 

 

Register

L

​

0

H

0

0

10V

Reset

Control

H

0

1

8V

0

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SUCCESSIVE APPROXIMATION ADC

+

-

 

 

 

DAC

 

Clock Generator

​

​

​

​

​

 

 

 

 

 

Register

H

​

0

H

0

0

0V

Reset

Control

H

0

1

8V

0

1

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SUCCESSIVE APPROXIMATION ADC

+

-

 

 

 

DAC

 

Clock Generator

​

​

​

​

​

 

 

 

 

 

Register

L

​

0

H

0

0

8V

Reset

Control

H

0

1

8V

0

1