ADC Resolution & Accuracy

Analog-to-Digital Converter (ADC) resolution can be used to describe the general performance of an ADC. Resolution and accuracy are terms that are often interchanged.

The resolution of an A/D converter (ADC) is specified in bits and determines how many distinct output codes (2n) the converter is capable of producing. In other words, resolution is the smallest voltage increment corresponding to a 1 LSB change. It's an important ADC specification because it determines the smallest analog input signal an ADC can resolve.

For example, an 8-bit ADC produces 28, or 256 output codes. The accuracy of the ADC determines how close the actual digital output is to the theoretically expected digital output for a given analog input. In other words, the accuracy of the converter determines how many bits in the digital output code represent useful information about the input signal. The accuracy of the ADC is a function of its internal circuitry and noise from external sources connected to the ADC input. In some cases, extra bits of resolution that are beyond the accuracy of the ADC can be beneficial.

ADC Effective Resolutions

Effective resolution describes the useful bits from an Analog to Digital conversion with respect with the input noise. Effective Number Of Bits (ENOB) is often used to specify ADC effective resolution. ENOB is NOT the same as the actual resolution of the ADC. Effective resolution is expressed using two units of measure: the specification of bits rms (0.707) refers to the output data; Effective resolution predicts the probability of a conversion level of repeatability of 70.7% for an input signal.

ENOB equation (based on an ideal ADC’s Signal-to-Noise Ratio (SNR):
SNR = (6.02) (ENOB) + 1.76 dB, where N is the ADC’s resolution.
ENOB = (SNR - 1.76)/6.02

Delta - Sigma A/D Converters Resolution

Delta-Sigma ADCs, for example, can provide resolutions as high as 24 bits. A given 24-bit Delta-Sigma converter may only provide 16 bits of accuracy. In this case, the 8 LSB’s represent random noise produced in the converter. However, these noise bits are used with digital filter algorithms to increase the useful measurement resolution at the expense of a lower sampling bandwidth.

Effective Number of Bits (ENOB) of a Delta-sigma ADC is a function of the ADC's resolution in bits and its standard deviation:
ENOB = N - log2(σ),
where σ = standard deviation of data, N = number of ADC bits

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