AD7771 datasheet(33/99 Pages) AD

33 / 99 page

AD7771

Data Sheet

Rev. 0 | Page 32 of 98

TERMINOLOGY

Common-Mode Rejection Ratio (CMRR)

CMRR is the ratio of the power in the ADC output at full-scale

frequency, f, to the power of a 100 mV p-p sine wave applied to

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

CMRR (dB) = 10 log(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.

DNL error is often specified in terms of resolution for which no

missing codes are guaranteed.

Integral Nonlinearity (INL) Error

Integral nonlinearity 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 a level 1½ LSB beyond

the last code transition. The deviation is measured 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

input signal to the rms noise measured for an input. The value

for dynamic range is expressed in decibels.

Channel to Channel Isolation

Channel to channel isolation is a measure of the level of

crosstalk between channels. It is measured by applying a full-scale

frequency sweep sine wave signal to all seven unselected input

channels and determining how much that signal is attenuated in

the selected channel. The value is given for worst case scenarios

across all eight channels of the AD7771.

Intermodulation Distortion

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 nor 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 AD7771 is tested using the CCIF standard, where

two input frequencies near the top end of the input bandwidth

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 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.

Gain Error

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

level ½ LSB above nominal negative full scale (−2.49999 V for the

±2.5 V range). The last transition (from 011 … 110 to 011 …

111) occurs for an analog voltage 1½ LSB below the nominal

full scale (2.49999 V for the ±2.5 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.

Gain Error Drift

Gain error drift is the ratio of the gain error change due to a

temperature change of 1°C and the full-scale range (2N). It is

expressed in ppm/°C.

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

LSB (V) =

REF

2×

The LSB referred to the input is

LSB (VIN) =

GAIN

REF

PGA

2×

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 + 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 including 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.

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