Following figure shows block diagram of Sigma Delta ADC.
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Sigma Delta ADC[/caption]
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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