AD7767 datasheet(14/24 Pages) AD

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AD7767

Rev. 0 | Page 14 of 24

TERMINOLOGY

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 harmonics to the fundamen-

tal. For the AD7767, it is defined as

()

THD

log

where:

V1 is the rms amplitude of the fundamental.

V2, V3, V4, V5, and V6 are the rms amplitudes of the second to

the sixth harmonics.

Nonharmonic Spurious-Free Dynamic Range (SFDR)

SFDR is the ratio of the rms signal amplitude to the rms value

of the peak spurious spectral component, excluding harmonics.

Dynamic Range

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

the rms noise measured with the inputs shorted together. The

value for the dynamic range is expressed in decibels.

Intermodulation Distortion

With inputs consisting of sine waves at two frequencies, fa

and fb, any active device with nonlinearities creates distortion

products at sum and difference frequencies of mfa ± 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 AD7767 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- 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.

Integral Nonlinearity (INL)

INL is the maximum deviation from a straight line passing

through the endpoints of the ADC transfer function.

Differential Nonlinearity (DNL)

DNL is the difference between the measured and the ideal

1 LSB change between any two adjacent codes in the ADC.

Zero Error

Zero error is the difference between the ideal midscale input

voltage (when both inputs are shorted together) and the actual

voltage producing the midscale output code.

Zero Error Drift

Zero error drift is the change in the actual zero error value due

to a temperature change of 1°C. It is expressed as a percentage

of full scale at room temperature.

Gain Error

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

for an analog voltage ½ LSB above the nominal negative full

scale. The last transition (from 011…110 to 011…111) should

occur for an analog voltage 1½ LSB below the nominal full

scale. 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 change in the actual gain error value due

to a temperature change of 1°C. It is expressed as a percentage

of full scale at room temperature.

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 a 100 mV sine wave

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

inputs at frequency fs.

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

where Pf is the power at the frequency f in the ADC output and

Pfs is the power at the frequency fs in the ADC output.

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