What is analog to digital converter- ADC using LM324 IC

The process of converting an analog voltage into an equivalent digital signal is known as Analog to Digital Conversion, abbreviated as ADC. An ADC is an electronic circuit which converts its analog input to corresponding binary value.The output depends up on the coding scheme followed in the ADC circuit. For example Analog value may convert to Gray code, excess 3 code and so on.

Analog to Digital converter ICs are also available to do this operation. Which reduce the circuit complexity such that a single IC capable of doing Analog to Digital Conversion.

The circuit below shows a 2 bit ADC circuit using LM324 comparator IC. A potential divider network and some combinational circuits are used for making this simple ADC.

LM324 best suited for Analog to Digital Converters because it has four embedded op amps, it require Vcc (5V) and ground only. No need of -Vcc like 741 op amp.

Components Required for ADC

  1. Resistors (1Kx4)
  2. IC LM324
  3. IC 7404
  4. IC 7432
  5. IC 7409

Design steps of Analog to Digital Converter

Truth Table of ADC

Truth Table of ADC

K-Maps for the design of ADC

design of ADC
design of ADC

Circuit Diagram of ADC using LM324

Analog to Digital Converter Circuit Diagram

Analog to Digital Converter Block Diagram

Block diagram of ADC explains the basic operation and signal flow,

Analog to Digital converter block diagram
  • Analog signal fed to the parallel combination of Comparators, it will produce encoded signal corresponding to input analog signal.
  • The encoded signal is then applied to Digital Code Converter (a combinational circuit), that will produce binary output.

Working of ADC Circuit

  • This is a simultaneous ADC, Simultaneous ADC is also called flash ADC and the speed of conversion is very fast.
  • Comparators continuously compare reference voltage at inverting terminal and analog voltage at non inverting terminal.
  • The reference voltage of each comparator is derived from potential divider network.

Reference voltage of lower comparator : Vcc (1/4) = Vcc/4
Reference voltage of middle comparator : Vcc (2/4) = Vcc/2
Reference voltage of upper comparator : Vcc (3/4)

  • If the analog input exceeds the reference voltage to any comparator, that comparator turns ON.
  • If all the comparators are OFF, the analog input signal will be between 0 and +Vcc/4.
  • When lower comparator ON and others are OFF, then input must be between Vcc/4 and Vcc/2.
  • For input voltage between +Vcc/2 and Vcc(3/4) , Lover and middle comparators are ON.
  • Above Vcc (3/4), all the three comparators will ON.
  • Thus the input analog voltage get converted in to encoded form with 3 output bits, but actually we need binary output like 00, 01, 10, and 11.
  • To represent 4 states in binary, only 2 bits are needed, so we are using a digital combinational code converter circuit with 3 logic gates. Thus it is possible to get binary outputs like 00, 01, 10, and 11.

What is the Resolution of ADC?

  • The term Resolution is used to describe the accuracy of ADC, resolution means the number of distinct values that ADC can generate over the range of analog values.
  • The output values are always in binary form, hence the resolution is typically expressed in bits. Consequently these outputs are the power of 2.
  • For example, an ADC with a resolution of 4 bits can encode an analog input to 16 different levels, since 24 = 16.
  • In our circuit we have 2 number of output bits, so the resolution is 22=4.
  • However the complexity of the useful circuit increases as resolution is increased.

Components Pin out

Pinout of 7404 NOT GatePinout of LM324 IC
Pinout of 7409 AND GatePinout of 7432 OR Gate
The circuit which does the revers operation that is conversion from Digital to Analog called as DAC circuit.

8 thoughts on “What is analog to digital converter- ADC using LM324 IC

  1. Hello may i ask why is that this circuit does not need any sampling pulse if it is an ADC circuit? Where do sampling takes place here? 🙂

  2. your truth table for “a” shows that it doesn’t turn on untill 3/4 vcc but in practice it would be the first amp to turn on.
    As the voltage increases then the second amp on the string would turn on and so on. the first would not turn off once the voltage is past the threshold as is the case for the second and all others in the string, am I missing something?
    My sim software won’t get the results you claim ?? Is this bcd ? for a good conversion shouldn’t 1 volt input equal a=5v
    b=0v, c=0v, 2 volts in = a =0 v, b= 5 v and c=0v 3 volts in = a=5 v b = 5 v, c= 0v. This is the correct bcd as I know it and
    have always used it.

  3. Hi,

    Can you explain the k -map ? I could not understand how could you list some combinations although they are not in the truth table ?

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