AD7761BSTZ datasheet(27/76 Pages) AD

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AD7761

Data Sheet

Rev. A | Page 26 of 75

TERMINOLOGY

AC Common-Mode Rejection Ratio (AC CMRR)

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

at frequency, f, to the power of a sine wave applied to the common-

mode voltage of AINx+ and AINx− at frequency, fS.

AC 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.

Gain Error

The first transition (from 100 … 000 to 100 … 001) occurs 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) occurs 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.

Gain Error Drift

Gain error drift is defined as the gain error change due to a

temperature change of 1°C. It is expressed in parts per million

per degree Celsius.

Integral Nonlinearity (INL) Error

INL 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 measured

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

Intermodulation Distortion (IMD)

With inputs consisting of sine waves at two frequencies, fa and

fb, any active device with nonlinearities creates distortion

products at the sum and difference frequencies of mfa and nfb,

where m, n = 0, 1, 2, 3, and so on. Intermodulation distortion

terms are those for which neither m or n are equal to 0. For

example, the second-order terms include (fa + fb) and (fa − fb),

and the third-order terms include (2fa + fb), (2fa − fb), (fa +

2fb), and (fa − 2fb).

The AD7761 is tested using the CCIF standard, in which two

input frequencies near to each other are used. In this case, the

second-order terms are usually distanced in frequency from the

original sine waves, and the third-order terms are usually at a

frequency close to the input frequencies.

As a result, the second-order and third-order terms are

specified separately. The calculation of the intermodulation

distortion is as per the THD specification, where it is the ratio

of the rms sum of the individual distortion products to the rms

amplitude of the sum of the fundamentals, expressed in

decibels.

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 as

follows:

LSB (V) = (2 × VREF)/2N

where:

VREF is the difference voltage between the REFx+ and REFx− pins.

N = 16.

Offset Error

Offset error is the difference between the ideal midscale input

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

output code.

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 the power

supply voltage from the nominal value.

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, between the rms amplitude

of the input signal and the peak spurious signal (excluding the

first five harmonics).

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.

AD7761BSTZ Datasheet(PDF) 27 Page - Analog Devices

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