Flash ADC or Parallel ADC and its Working Principle

Another type of ADC is parallel ADC. Parallel ADC is called as Flash ADC. Its response is very fast. it converts analog signal into digital signal using parallel set of comparators. As its conversion time is very fast it is called as flash ADC.

Following figure shows circuit diagram of parallel ADC or flash ADC.

Flash ADC or Parallel ADC
Flash ADC or Parallel ADC

n-bit Flash ADC consist of parallel combination of 2^n-1 comparators. Outputs of all comparators are connected to an encoder.

Working Principle of flash ADC

Analog voltage is applied to non inverting terminals of all comparators using a single line. Reference voltage is applied to inverting terminals of comparators using divider circuit.

Each comparator produces digital output in the form of 1 or 0. If unknown analog voltage is greater than reference voltage comparator produces high logic. If analog voltage is less than reference voltage then comparator produces low logic i.e. 0.

Thus all parallel comparator produces digital representation of analog voltage in the form of zero and one. These outputs of comparator are then applied to the fast encoder. Encoder converts those zeros and once into binary number and produces digital binary output.

For example, see below table. When unknown voltage is 5 i.e. lies between 4.375 &5.625 is applied to the flash ADC, first four encoders produces output ‘1’ and last three encoders produces output ‘0’. Encoder converts this ‘1111000’ comparator output into ‘100’ binary number as digital output.

Table shows the outputs of comparators and encoder for a 3 bit flash ADC. The range of operation is given as 0-10V.

Analog input

(V)

Comparator Output Encoder Output
C1 C2 C3 C4 C5 C6 C7 D2 D1 D0
0.000-0.625 0 0 0 0 0 0 0 0 0 0
0.625-1.875 1 0 0 0 0 0 0 0 0 1
1.875-3.125 1 1 0 0 0 0 0 0 1 0
3.125-4.375 1 1 1 0 0 0 0 0 1 1
4.375-5.625 1 1 1 1 0 0 0 1 0 0
5.625-6.875 1 1 1 1 1 0 0 1 0 1
6.875-8.125 1 1 1 1 1 1 0 1 1 0
8.125-10.000 1 1 1 1 1 1 1 1 1 1

 

As the number of bits of ADC increases its resolution increases. But such high bit converter is bulky and expensive.

Also see: Sigma Delta ADC or noise eliminating ADC.

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Sigma Delta ADC & its Working Principle

Hi friends, today we will see Sigma Delta ADC and its working. Sigma Delta ADC is widely used in communication system, professional audio system and high precision measurement system. Sigma Delta ADC has characteristics like, high resolution, low cost and low conversion speed.

Following figure shows block diagram of Sigma Delta ADC.

Sigma Delta ADC
Sigma Delta ADC

Sigma Delta ADC consists of two main blocks, Sigma Delta modulator and digital decimeter. Sigma Delta modulator consist of difference amplifier, integrator, comparator and 1-bit DAC. Digital decimeter is used for digital filtering and down sampling.

Working Principle

Analog signal which is to be digitized is applied to the non inverting terminal of difference amplifier. Inverting terminal of difference amplifier is connected with either +Vref or –Vref depends on output of 1-bit DAC. Output of difference amplifier is integrated using integrator as shown in diagram.

Output of integrator is applied to the non inverting terminal of comparator. In this case comparator works as 1-bit ADC and produces output as 1 or 0.

Output of comparator is connected to the 1-bit DAC. DAC is used to connect either +Vref or –Vref to the inverting terminal of difference amplifier.

DACs output is them again subtracted from analog input. This process is continuous in closed loop. After each loop 1-bit ADC produces 1 or 0. Density of ‘1’ depends on analog voltage supplied. If analog voltage is high then density will be high and if analog signal is low density of ‘1’ will be low.

Output of 1-bit ADC is also connected to the digital decimeter. It is used for digital filtering and down sampling. It produces n-bit digital output in binary format.

Advantages of Sigma Delta ADC

  • Sigma Delta ADC is inexpensive since all circuitry within the converter is digital.
  • The output of sigma delta ADC is inherently linear but it has little differential non linearity.
  • It do not require sample and hold circuit. It is because due to high sampling rate and low precision.

Disadvantages

  • It is limited to high resolution and very low frequency applications.
  • It takes quite long time for producing first digital output because of digital filtering and down sampling.
  • It is not possible to use Sigma Delta ADC for multiplexed ac input signals.

Note: Sigma Delta ADC is also known as Delta Sigma ADC, oversampling ADC or noise shaping ADC.

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Weighted Resistor DAC and its Operation

HI friends, today we are going to learn weighted resistor DAC. Many times we require to convert a digital output into its equivalent analog voltage. To perform this operation we are going to use weighted resistor digital to analog converter.

The following figure shows the circuit diagram of weighted resistor DAC. This DAC circuit uses weighted values of resistor like 2R, 4R, 6R, 8R and so on depending on the digital inputs available therefore such type of network is known as weighted resistor DAC.

Weighted Resistor DAC Circuit
Weighted Resistor DAC Circuit

This circuit consists of a transistor switch (shown by the upward arrow) which turns on the switch when the digital input is ‘1’ and if digital input becomes ‘0’ it will open the switch. When transistor switch gets closed, current flows through the weighted resistor due to the reference voltage as shown in circuit diagram.

When all such currents from different weighted resistors get added at summing point (which is also known as a virtual ground) of the operational amplifier it will produce a proportional voltage as its output.

For a 4 bit DAC, the output V0 is given as follows

Where S3, S2, S1 and S0 represents the status of the switches i.e. on or off (1 or 0).

If resistors are in binary weights i.e. R3=2Rf, R2=4Rf, R1=8Rf and R0=16Rf, the above equation can be written as,

From the above discussion, we can say that for a 4 bit DAC 4 switches produces 16 different combinations of output and hence produces 16 different output voltage.in general n-bit DAC produces 2^n different discrete analog voltages.

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