ADCAnalogue to digital converter converts the analogue signal to digital values by sampling. These digital values represent the analogue signal that a processor unit can process. Analogue to Digital Converters are the primary elements of data acquisition systems. Besides DAQs, ADCs are used in signal processing, communication, and instrumentation.

ADC Form Factors

A / D Converters are available in a variety of form factors. These include integrated circuits (ICs) or PCBs, chassis mount, rack mount, DIN rails, slots, modules, and stand-alone devices.

ADC Specifications

An analogue to digital converter is specified by the Resolution it provides, sampling frequency, inputs, bandwidth, and accuracy.

ADC Resolution

Resolution of an ADC is defined by the number of bits it uses to represent an analogue signal. Greater the number of bits higher will be the resolution. In fact each additional bit doubles the resolution. But increasing the bits may cause certain problems such as aliasing (signal overlapping). For instance an analogue signal of 0-10 Volts can be represented using 10 bits as well as 12 bits. But with 10 bits analogue signal is represented in .01 increments (210 = 1024 and 0.01 * 1024 = 10.24). Similarly with 12 bits it is represented in .0024 divisions. In second case there is every chance of signals overlapping while converting back to analogue signal.

ADC Inputs

The inputs for an ADC are differential channels. These channels either use a combination of two single ended inputs or the difference of two signals as analogue input. In case of difference the differential channels filter out the common mode.

Errors and Accuracy

DNL And INL errors are most important to consider selecting an ADC. Differential nonlinearity (DNL) error refers to the difference between measured and ideal digital output. Integrated non-linearity (INL) error is the difference between ideal and measured transfer function (output function). Moreover SNR (signal to noise) ratios and SNAD (signal to noise distortion) ratios are also affects the performance of an ADC. Accuracy is measured as percentage value. It shows how correct the output value is measured. Temperature, signal conditioning linearity, and hysteresis are the parameters that can affect the accuracy. Overall performance of ADC also depends upon sampling rate, input voltage range, and other related variables.

ADC Chip Architectures

ADC Chips are available in several types depending upon the various architectures associated with them. These include SAR (successive approximation registers) architecture, flash architecture, pipeline architecture, sub-ranging approach, and sigma delta architecture. These architectures have benefits and drawbacks over each other. For example pipeline architecture removes the limitation issues associated with FLASH and SAR architectures. Sub-ranging approach adds the benefits of SAR, FLASH, and pipeline architectures. Sigma-delta architecture is the different of all. It consists of a DAC, an integrator, and a comparator.