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Data Sheet

AD7981

Rev. B | Page 13 of 26

TERMINOLOGY

Integral Nonlinearity (INL)

INL refers to the deviation of each individual code from a line

drawn from negative full scale through positive full scale. The

point used as negative full scale occurs ½ LSB before the first

code transition. Positive full scale is defined as a level 1½ LSB

beyond the last code transition. The deviation is measured from

the middle of each code to the true straight line (see Figure 37).

Differential Nonlinearity (DNL)

In an ideal ADC, code transitions are 1 LSB apart. DNL is the

maximum deviation from this ideal value. It is often specified in

terms of resolution for which no missing codes are guaranteed.

Zero Error

The first transition occurs at a level ½ LSB above analog ground

(38.1 µV for the 0 V to 5 V range). The offset error is the

deviation of the actual transition from that point.

Gain Error

The last transition (from 111 … 10 to 111 … 11) occurs for an

analog voltage 1½ LSB below the nominal full scale (4.999886 V

for the 0 V to 5 V range). The gain error is the deviation of the

actual level of the last transition from the ideal level after the

offset is adjusted out.

Spurious-Free Dynamic Range (SFDR)

SFDR is the difference, in decibels (dB), between the rms

amplitude of the input signal and the peak spurious signal.

Effective Number of Bits (ENOB)

ENOB is a measurement of the resolution with a sine wave

input. It is related to SINAD by the following formula and is

expressed in bits:

ENOB = (SINADdB − 1.76)/6.02

Noise Free Code Resolution

Noise free code resolution is the number of bits beyond which it

is impossible to distinctly resolve individual codes. It is

calculated as follows and is expressed in bits:

Noise Free Code Resolution = log2(2N/Peak-to-Peak Noise)

Effective Resolution

Effective resolution is calculated as follows and is expressed in

bits:

Effective Resolution = log2(2N/RMS Input Noise)

Total Harmonic Distortion (THD)

THD is the ratio of the rms sum of the first five harmonic

components to the rms value of a full-scale input signal and is

expressed in dB.

Dynamic Range

Dynamic range is the ratio of the rms value of the full scale to

the total rms noise measured with the inputs shorted together. It

is measured with a signal at −60 dBFS to include all noise sources

and DNL artifacts. The value for dynamic range is expressed in dB.

Signal-to-Noise Ratio (SNR)

SNR is the ratio of the rms value of the actual input signal to the

rms sum of all other spectral components below the Nyquist

frequency, excluding harmonics and dc. The value for SNR is

expressed in dB.

Signal-to-Noise-and-Distortion (SINAD) Ratio

SINAD is the ratio of the rms value of the actual input signal to

the rms sum of all other spectral components below the Nyquist

frequency, including harmonics but excluding dc. The value for

SINAD is expressed in dB.

Aperture Delay

Aperture delay is the measure of the acquisition performance. It

is the time between the rising edge of the CNV input and when

the input signal is held for a conversion.

Transient Response

Transient response is the time required for the ADC to

accurately acquire its input after a full-scale step function is

applied.

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