AD7655AST datasheet(11/28 Pages) AD

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AD7655

Rev. B | Page 11 of 28

TERMINOLOGY

Integral Nonlinearity Error (INL)

Linearity error 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 LSBs beyond the last code transition. The deviation is

measured from the middle of each code to the true straight line.

Differential Nonlinearity Error (DNL)

In an ideal ADC, code transitions are 1 LSB apart. Differential

nonlinearity is the maximum deviation from this ideal value,

and is often specified in terms of resolution for which no

missing codes are guaranteed.

Full-Scale Error

The last transition (from 111. . .10 to 111. . .11) should occur for

an analog voltage 1½ LSBs below the nominal full scale

(4.999886 V for the 0 V to 5 V range). The full-scale error is the

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

level.

Unipolar Zero Error

The first transition should occur at a level ½ LSB above analog

ground (76.29 μV for the 0 V to 5 V range). The unipolar zero

error is the deviation of the actual transition from that point.

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 decibels.

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 decibels.

Signal-to-Noise and Distortion Ratio (SINAD)

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 decibels.

Spurious-Free Dynamic Range (SFDR)

The difference, in decibels, 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 and expressed in bits by

ENOB = (SINADdB − 1.76)/6.02

Aperture Delay

Aperture delay is a measure of acquisition performance and is

measured from the falling edge of the CNVST input to when

the input signals are held for a conversion.

Transient Response

The time required for the AD7655 to achieve its rated accuracy

after a full-scale step function is applied to its input.

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