AD7298 datasheet(13/25 Pages) AD

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AD7298

Rev. B | Page 12 of 24

TERMINOLOGY

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 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

digitization process; the more levels, the smaller the quantization

noise. The theoretical signal-to-noise and 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, the SINAD is 74 dB for an ideal 12-bit converter.

Total Harmonic Distortion (THD)

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

the AD7298, it is defined as

log

)

(

THD

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

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 specification

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.

Integral Nonlinearity

The maximum deviation from a straight line passing through

the endpoints of the ADC transfer function. The endpoints 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

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, GND1 + 1 LSB.

Offset Error Match

The difference in offset error between any two channels.

Gain Error

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

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

offset error has been adjusted out.

Gain Error Matching

The difference in gain error between any two channels.

Track-and-Hold Acquisition Time

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

of conversion. Track-and-hold acquisition time is the time

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

its final value, within ±1 LSB, after the end of conversion.

Power Supply Rejection Ratio (PSRR)

PSRR is defined as 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 ADC VDD supply of frequency, fS. The frequency

of the input varies from 5 kHz to 25 MHz.

PSRR (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.

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