AD7468BRT-R datasheet(16/28 Pages) AD

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AD7466/AD7467/AD7468

Rev. C | Page 16 of 28

TERMINOLOGY

Integral Nonlinearity (INL)

The maximum deviation from a straight line passing through

the endpoints of the ADC transfer function. For the AD7466/

AD7467/AD7468, the endpoints of the transfer function are

zero scale, a point 1 LSB below the first code transition, and full

scale, a point 1 LSB above the last code transition.

Differential Nonlinearity (DNL)

The difference between the measured and the ideal 1 LSB

change between any two adjacent codes in the ADC.

Offset Error

The deviation of the first code transition (00 . . . 000) to

(00 . . . 001) from the ideal (that is, AGND + 1 LSB).

Gain Error

The deviation of the last code transition (111 . . . 110) to

(111…111) from the ideal (that is, VREF − 1 LSB) after the offset

error has been adjusted out.

Track-and-Hold Acquisition Time

The time required for the part to acquire a full-scale step

input value within ±1 LSB, or a 30 kHz ac input value within

±0.5 LSB. The AD7466/AD7467/AD7468 enter track mode on

the CS falling edge, and return to hold mode on the third SCLK

falling edge. The parts remain in hold mode until the following

CS falling edge. See Figure 3 and the Serial Interface section for

more details.

Signal-to-Noise Ratio (SNR)

The measured ratio of signal to noise at the output of the ADC.

The signal is the rms value of the sine wave input. Noise is the

rms quantization error within the Nyquist bandwidth (fS/2).

The rms value of the sine wave is half of its peak-to-peak value

divided by √2, and the rms value for the quantization noise is

q/√12. The ratio depends on the number of quantization levels

in the digitization process; the more levels, the smaller the

quantization noise.

For an ideal N-bit converter, the SNR is defined as

SNR = 6.02 N + 1.76 db

Thus, for a 12-bit converter, it is 74 dB; for a 10-bit converter, it

is 62 dB; and for an 8-bit converter, it is 50 dB.

However, in practice, various error sources in the ADCs cause

the measured SNR to be less than the theoretical value. These

errors occur due to integral and differential nonlinearities,

internal ac noise sources, and so on.

Signal-to-Noise and Distortion Ratio (SINAD)

The measured ratio of signal-to-noise and distortion at the

output of the ADC. The signal is the rms value of the sine wave,

and noise is the rms sum of all nonfundamental signals up to

half the sampling frequency (fS/2), including harmonics, but

excluding dc.

Total Unadjusted Error (TUE)

A comprehensive specification that includes gain error, linearity

error, and offset error.

Total Harmonic Distortion (THD)

The ratio of the rms sum of harmonics to the fundamental. For

the AD7466/AD7467/AD7468, it is defined as

()

THD

log

where V1 is the rms amplitude of the fundamental, and V2, V3,

V4, V5, and V6 are the rms amplitudes of the second through

sixth harmonics.

Peak Harmonic or Spurious Noise (SFDR)

The ratio of the rms value of the next-largest component in the

ADC output spectrum (up to fS/2 and excluding dc) to the rms

value of the fundamental. Typically, the value of this specifica-

tion is determined by the largest harmonic in the spectrum, but

for ADCs where the harmonics are buried in the noise floor, it

is a noise peak.

Intermodulation Distortion (IMD)

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 zero. For

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

while the third-order terms include (2fa + fb), (2fa – fb),

(fa + 2fb), and (fa − 2fb).

The AD7466/AD7467/AD7468 are tested using the CCIF

standard where two input frequencies are used. In this case,

the second-order terms are usually distanced in frequency from

the original sine waves, while 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 dB.

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