AD7477AARM datasheet(11/24 Pages) AD

11 / 24 page

REV. C

AD7476A/AD7477A/AD7478A

–11–

TERMINOLOGY

Integral Nonlinearity

This is the maximum deviation from a straight line passing through

the endpoints of the ADC transfer function. For the AD7476A/

AD7477A/AD7478A, 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

This is the difference between the measured and the ideal 1 LSB

change between any two adjacent codes in the ADC.

Offset Error

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

(00 . . . 001) from the ideal, i.e., AGND + 1 LSB.

Gain Error

This is the deviation of the last code transition (111 . . . 110)

to (111 . . . 111) from the ideal, i.e., VREF – 1 LSB after the

offset error has been adjusted out.

Track-and-Hold Acquisition Time

The track-and-hold amplifier returns to track mode at the end

of conversion. The track-and-hold acquisition time is the time

required for the output of the track-and-hold amplifier to reach its

final value, within

±0.5 LSB, after the end of conversion. See

the Serial Interface section for more details.

Signal-to-(Noise + Distortion) Ratio (SINAD)

This is the measured ratio of signal-to-(noise + distortion) at the

output of the A/D converter. The signal is the rms amplitude of

the fundamental. Noise is the sum of all nonfundamental signals

up to half the sampling frequency (fS/2), excluding dc. The ratio

is dependent on the number of quantization levels in the digiti-

zation process; the more levels, the smaller the quantization

noise. The theoretical signal-to-(noise + distortion) ratio for an

ideal N-bit converter with a sine wave input is given by

Signal-to-(Noise + Distortion) = (6.02 N + 1.76) dB

Thus, it is 74 dB for a 12-bit converter, 62 dB for a 10-bit con-

verter, and 50 dB for an 8-bit converter.

Total Unadjusted Error (TUE)

This is a comprehensive specification that includes the gain,

linearity, and offset errors.

Total Harmonic Distortion (THD)

Total harmonic distortion is the ratio of the rms sum of har-

monics to the fundamental. It is defined as

THD

VVV

() =

+++

20 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 the

sixth harmonics.

Peak Harmonic or Spurious Noise (SFDR)

Peak harmonic or spurious noise is defined as 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. Normally, the value of this specification is deter-

mined by the largest harmonic in the spectrum. But for ADCs

where the harmonics are buried in the noise floor, it will be a

noise peak.

Intermodulation Distortion (IMD)

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

any active device with nonlinearities will create distortion products

at sum and difference frequencies of mfa

± nfb, where m and

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 AD7476A/AD7477A/AD7478A are tested using the CCIF

standard where two input frequencies are used (see fa and fb on

the specification pages). 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 dis-

tortion 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 dBs.

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