AD7985BCPZ datasheet(13/29 Pages) AD

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AD7985

Data Sheet

Rev. C | Page 12 of 28

TERMINOLOGY

Aperture Delay

Aperture delay is the 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.

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.

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.

The value for dynamic range is expressed in decibels. It is mea-

sured with a signal at −60 dBFS so that it includes all noise

sources and DNL artifacts.

Effective Number of Bits (ENOB)

ENOB is a measurement of the resolution with a sine wave

input. It is expressed in bits and is related to SINAD as follows:

ENOB = (SINADdB − 1.76)/6.02

Effective Resolution

Effective resolution is expressed in bits and is calculated as follows:

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

Gain Error

The last transition (from 111 … 10 to 111 … 11) must 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

difference between the actual level of the last transition and the

actual level of the first transition from the difference between

the ideal levels.

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 22).

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 expressed

in bits and is calculated as follows:

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

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.

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)

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

amplitude of the input signal and the peak spurious signal.

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.

Transient Response

Transient response is the time required for the ADC to accurately

acquire the input after a full-scale step function is applied.

Zero Error

Zero error is the difference between the ideal midscale voltage,

that is, 0 V, from the actual voltage producing the midscale

output code, that is, 0 LSB.

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