AD7643 datasheet(12/24 Pages) AD

12 / 24 page

AD7625

Data Sheet

Rev. A | Page 12 of 24

TERMINOLOGY

Common-Mode Rejection Ratio (CMRR)

CMRR is defined as the ratio of the power in the ADC output at

full-scale frequency, f, to the power of an 80 mV p-p sine wave

applied to the common-mode voltage of VIN+ and VIN− at

frequency fS.

CMRR (dB) = 10log(Pf/PfS)

where:

Pf is the power at frequency f in the ADC output.

PfS is the power at frequency fS in the ADC output.

Differential Nonlinearity (DNL) Error

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.

Integral Nonlinearity (INL) Error

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 LSB beyond the last code transition. The deviation is meas-

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

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.

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]

Gain Error

The first transition (from 100 … 000 to 100 …001) should occur

at a level ½ LSB above nominal negative full scale (−4.0959375 V

for the ±4.096 V range). The last transition (from 011 … 110 to

011 … 111) should occur for an analog voltage 1½ LSB below

the nominal full scale (+4.0959375 V for the ±4.096 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.

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

(V) =

Power Supply Rejection Ratio (PSRR)

Variations in power supply affect the full-scale transition but not

the linearity of the converter. PSRR is the maximum change in

the full-scale transition point due to a change in power supply

voltage from the nominal value.

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) meas-

ured 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.

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 + 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 decibels.

Spurious-Free Dynamic Range (SFDR)

SFDR is the difference, in decibels, 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.

Zero Error

Zero error is the difference between the ideal midscale input

voltage (0 V) and the actual voltage producing the midscale

output code.

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