Analog-to-Digital Converters

Analog-to-digital converters (ADC) sample an analog signal and convert it to a series of digital values to represent the signal to a computer processor.  There are various techniques of A/D conversion, including flash, half flash, integrator, delta sigma or modulator, successive approximation, and voltage-to-frequency.  A flash ADC applies the input in parallel to many fast comparators whose thresholds are equally spaced throughout the desired input voltage range, typically 1V. At any moment, all the comparators that have thresholds below the input voltage are on, and the rest are off.   Half-flash analog-to-digital converters determine their output code by digitally combining the results of two sequentially performed, lower-resolution flash conversions.  An integrator is an ADC whose output code represents the average value of the input voltage over a given time interval.  Delta-Sigma is a high-accuracy circuit that samples at a higher rate and lower resolution than is needed and (by means of feedback loops) pushes the quantization noise above the frequency range of interest. Successive approximation analog-to-digital converters sequentially compares a series of binary-weighted values with an analog input to produce an output digital word in n steps, where n is the bit resolution of the ADC.  Voltage-to-frequency analog-to-digital converters convert an input voltage to an output pulse train with a frequency proportional to the input voltage. Counting pulses determine output frequency over a fixed time interval and the voltage is inferred from the known relationship.

Important device specifications to consider when searching for analog-to-digital converters include differential analog input channels, sampling frequency, resolution, bandwidth, and accuracy.  Differential channels use the difference between two signals as an input; common mode is filtered out. In some systems, differential inputs are combinations of two single-ended inputs; in this case, twice the number of differential channels would be available as single-ended inputs.  Sampling frequency is the frequency of analog signal sampling and conversion to a digital value.  Resolution refers to the degree of fineness of the digital word representing the analog value. A ten-bit number contains 2¹⁰, or 1024, increments. A 0-10V signal could therefore be resolved into approximately 0.01V increments. A 12-bit representation would be in 2¹² (4096) increments, or divisions of 0.0024V for the same signal. Each additional bit doubles the resolution, and one bit is required for the polarity (sign) of a number.  Bandwidth is the difference between the high and low limits of the frequency response, typically defined by a variation from a nominal value by a stated value such as 3 dB.  Accuracy depends on the signal conditioning linearity, hysteresis, temperature considerations, etc. It is represented here as percent full scale of measurement range.

Other important specifications to consider when searching for analog-to-digital converters include signal inputs, form factors, and computer bus.  Choices for signal inputs include DC voltage input, DC current input, AC voltage input, and AC current input.  Common form factors are IC or board mount, circuit board, panel or chassis mount, modular bay or slot system, rack mount, DIN rail, and stand-alone.

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