AD8021 datasheet(9/28 Pages) AD

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AD7685

Rev. C | Page 9 of 28

TERMINOLOGY

Integral Nonlinearity Error (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 26).

Differential Nonlinearity Error (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.

Offset Error

The first transition should occur 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) should

occur 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)

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

ENOB = (SINADdB − 1.76)/6.02

and is expressed in bits.

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.

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 + Distortion), 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 dB.

Aperture Delay

Aperture delay is a measure of the acquisition performance and

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

the input signal is held for a conversion.

Transient Response

The time required for the ADC to accurately acquire its input

after a full-scale step function is applied.

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