AD7294 datasheet(20/48 Pages) AD

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Data Sheet

AD7294

Rev. I | Page 19 of 47

TERMINOLOGY

DAC TERMINOLOGY

Relative Accuracy

For the DAC, relative accuracy or integral nonlinearity (INL) is

a measure of the maximum deviation, in LSBs, from a straight

line passing through the endpoints of the DAC transfer function.

Differential Nonlinearity

Differential nonlinearity (DNL) is the difference between the

measured change and the ideal 1 LSB change between any two

adjacent codes. A specified differential nonlinearity of ±1 LSB

maximum ensures monotonicity. This DAC is guaranteed mono-

tonic by design.

Zero Code Error

Zero code error is a measure of the output error when zero code

(0x0000) is loaded to the DAC register. Ideally, the output

should be 0 V. The zero code error is always positive in the

AD7294 because the output of the DAC cannot go below 0 V.

Zero code error is expressed in mV.

Full-Scale Error

Full-scale error is a measure of the output error when full-scale

code (0xFFFF) is loaded to the DAC register. Ideally, the output

should be VDD − 1 LSB. Full-scale error is expressed in mV.

Gain Error

Gain error is a measure of the span error of the DAC. It is the

deviation in slope of the DAC transfer characteristic from ideal,

expressed as a percent of the full-scale range.

Total Unadjusted Error

Total unadjusted error (TUE) is a measure of the output error,

taking all of the various errors into account.

Zero Code Error Drift

Zero code error drift is a measure of the change in zero code

error with a change in temperature. It is expressed in µV/°C.

Gain Error Drift

Gain error drift is a measure of the change in gain error with

changes in temperature. It is expressed in (ppm of full-scale

range)/°C.

ADC 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 AD7294, 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, AGND + 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 Match

The difference in gain error between any two channels.

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