AD7631BSTZ datasheet(16/32 Pages) AD

16 / 32 page

AD7631

Rev. A | Page 16 of 32

TERMINOLOGY

Least Significant Bit (LSB)

The least significant bit, or LSB, is the smallest increment that

can be represented by a converter. For a fully differential input

ADC with N bits of resolution, the LSB expressed in volts is

INp-p

LSB

)

(

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 a 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. It

is often specified in terms of resolution for which no missing

codes are guaranteed.

Bipolar Zero Error

The difference between the ideal midscale input voltage (0 V)

and the actual voltage producing the midscale output code.

Unipolar Offset Error

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

ground. The unipolar offset error is the deviation of the actual

transition from that point.

Full-Scale Error

The last transition (from 111…10 to 111…11 in straight binary

format) should occur for an analog voltage 1½ LSB below the

nominal full scale. The full-scale error is the deviation in LSB

(or % of full-scale range) of the actual level of the last transition

from the ideal level and includes the effect of the offset error.

Closely related is the gain error (also in LSB or % of full-scale

range), which does not include the effects of the offset error.

Dynamic Range

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

the rms noise measured for an input typically at −60 dB. The

value for dynamic range is expressed in decibels.

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 + 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 (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 and is expressed in bits by

ENOB = [(SINADdB − 1.76)/6.02]

Aperture Delay

Aperture delay is a measure of the acquisition performance

measured from the falling edge of the CNVST input to when

the input signal is held for a conversion.

Transient Response

The time required for the AD7631 to achieve its rated accuracy

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

Reference Voltage Temperature Coefficient

The reference voltage temperature coefficient is derived from

the typical shift of output voltage at 25°C on a sample of parts at

the maximum and minimum reference output voltage (VREF)

measured at TMIN, T (25°C), and TMAX. It is expressed in ppm/°C as

(

ppm/

)

–

(

)

(

)

Min

–

)

Max

)

(

TCV

MIN

MAX

REF

where:

VREF (Max) = maximum VREF at TMIN, T (25°C), or TMAX.

VREF (Min) = minimum VREF at TMIN, T (25°C), or TMAX.

VREF (25°C) = VREF at 25°C.

TMAX = +85°C.

TMIN = –40°C.

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