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AD7321

Data Sheet

Rev. B | Page 14 of 36

TERMINOLOGY

Differential Nonlinearity

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

change between any two adjacent codes in the ADC.

Integral Nonlinearity

This is the maximum deviation from a straight line passing

through the endpoints of the ADC transfer function. 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).

Offset Code Error

This applies to straight binary output coding. It is the deviation

of the first code transition (00 ... 000) to (00 ... 001) from the

ideal, that is, AGND + 1 LSB.

Offset Error Match

This is the difference in offset error between any two input

channels.

Gain Error

This applies to straight binary output coding. It is the deviation

of the last code transition (111 ... 110) to (111 ... 111) from the

ideal (that is, 4 × VREF − 1 LSB, 2 × VREF − 1 LSB, VREF −1 LSB)

after adjusting for the offset error.

Gain Error Match

This is the difference in gain error between any two input

channels.

Bipolar Zero Code Error

This applies when using twos complement output coding and a

bipolar analog input. It is the deviation of the midscale

transition (all 1s to all 0s) from the ideal input voltage, that is,

AGND − 1 LSB.

Bipolar Zero Code Error Match

This refers to the difference in bipolar zero code error between

any two input channels.

Positive Full-Scale Error

This applies when using twos complement output coding and

any of the bipolar analog input ranges. It is the deviation of the

last code transition (011 ... 110) to (011 ... 111) from the ideal

(4 × VREF − 1 LSB, 2 × VREF − 1 LSB, VREF − 1 LSB) after

adjusting for the bipolar zero code error.

Positive Full-Scale Error Match

This is the difference in positive full-scale error between any

two input channels.

Negative Full-Scale Error

This applies when using twos complement output coding and

any of the bipolar analog input ranges. This is the deviation of

the first code transition (10 ... 000) to (10 ... 001) from the ideal

(that is, −4 × VREF + 1 LSB, −2 × VREF + 1 LSB, −VREF + 1 LSB)

after adjusting for the bipolar zero code error.

Negative Full-Scale Error Match

This is the difference in negative full-scale error between any

two input channels.

Track-and-Hold Acquisition Time

The track-and-hold amplifier returns into track mode after the

14th SCLK rising edge. Track-and-hold acquisition time is the

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

reach its final value, within ±½ LSB, after the end of a conversion.

For the ±2.5 V range, the specified acquisition time is the time

required for the track-and-hold amplifier to settle to within ±1 LSB.

Signal to (Noise + Distortion) Ratio

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

output of the ADC. The signal is the rms amplitude of the

fundamental. Noise is the sum of all non-fundamental signals up

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

dependent on the number of quantization levels in the digi-

tization process. The more levels, the smaller the quantization

noise. Theoretically, the 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

For a 13-bit converter, this is 80.02 dB.

Total Harmonic Distortion

Total harmonic distortion (THD) is the ratio of the rms sum of

harmonics to the fundamental. For the AD7321 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 the

sixth harmonics.

Peak Harmonic or Spurious Noise

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, excluding dc) to the rms value of the

fundamental. Normally, 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, the

largest harmonic could be a noise peak.

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