Analog-to-digital converters

 

There has been a drastic rise in analog-to-digital converters recently due to almost all daily life appliances' digitalization. As the name suggests, analog-to-digital converters convert analog signals such as temperature, voltage, current, into digital representations of these signals. These signals are fed into a controller that reads, evaluates, and performs various functions depending on the signals. 

Analog-to-digital converters (ADC) work by carrying out two processes: quantization and sampling. The ADC converts an analog signal with theoretically infinite resolution into a digital code with a finite resolution. This digital code appears in a binary code format on the converter's output. The analog input signal falls between the quantization levels this process results in a quantization error. This error determines the analog-to-digital converter's maximum dynamic range. The performance limits of an ideal ADC are selected through sampling and quantization. The two measures of analog-to-digital converters performance are speed and accuracy. The ADC's are categorized based on these parameters. General-purpose ADCs have 8- to 14-bit resolution and conversion rates below 10 Msamples/s. High-speed ADC's have conversion rates above 10 Msamples/s, while those with 16 bits or more fall into the precision ADC category. Depending on the architecture, analog-to-digital converters are categorized into the following common type: Delta Sigma, Pipeline, Successive approximation (SAR). For any given type of ADC the higher the speed the lower its resolution. 

  • Flash type ADC's have a max sample rate of 10 gigasamples/s and a resolution of 4-12 bits. 
  • SAR type ADC's have a max sample rate of 10 megasamples/s and a resolution of 8-18 bits. 
  • Delta sigma-type ADC's have a max sample rate of 1 megasamples/s and a resolution of 8-32 bits
  • Whereas pipelined type ADC's have a max sample rate of 1 gigasamples/s and a resolution of 8-16 bits. 

It is difficult to choose the right ADC for the job. The four important variables to consider while selecting an ADC are; Resolution, accuracy, speed, and noise. You can further optimize your selection by considering relatively less important variables such as input voltage, interface, and the number of channels. Keeping in view the practicality variables such as physical size, power requirements, and possible inputs can also come into play.

 Pipeline ADC manages to achieve high resolution and high speed because they combine the best features of Flash type and SARs ADC. Pipeline ADC's are used in various applications such as ultrasonic medical imaging and digital videos. While Flash-type ADC is expensive and larger, their high speed enables them to deal with huge amounts of data, such as converting analog video to digital. The majority of SARs usage is in instrumentation and data acquisition applications where accuracy reigns supreme. Delta Sigma type ADCs offer very high accuracy; thus, they are largely employed for analog-to-digital audio conversion and instrumentation.

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