2

1

f

2

6

f

2

2

f

10

‡

A

A

+

+

A

log

20

=

THD

ADC124S021

SNAS277F – MARCH 2005 – REVISED MARCH 2013

www.ti.com

LSB), after adjusting for offset error.

INTEGRAL NON-LINEARITY (INL) is a measure of the deviation of each individual code from a line drawn from

negative full scale (½ LSB below the first code transition) through positive full scale (½ LSB above the last

code transition). The deviation of any given code from this straight line is measured from the center of that

code value.

INTERMODULATION DISTORTION (IMD) is the creation of additional spectral components as a result of two

sinusoidal frequencies being applied to the ADC input at the same time. It is defined as the ratio of the

power in the second and third order intermodulation products to the power in one of the original

frequencies. IMD is usually expressed in dB.

MISSING CODES are those output codes that will never appear at the ADC outputs. These codes cannot be

reached with any input value. The ADC124S021 is ensured not to have any missing codes.

OFFSET ERROR is the deviation of the first code transition (000...000) to (000...001) from the ideal (i.e. GND +

0.5 LSB).

SIGNAL TO NOISE RATIO (SNR) is the ratio, expressed in dB, of the rms value of the input signal to the rms

value of the sum of all other spectral components below one-half the sampling frequency, not including

d.c. or the harmonics included in THD.

SIGNAL TO NOISE PLUS DISTORTION (S/N+D or SINAD) Is the ratio, expressed in dB, of the rms value of

the input signal to the rms value of all of the other spectral components below half the clock frequency,

including harmonics but excluding d.c.

SPURIOUS FREE DYNAMIC RANGE (SFDR) is the difference, expressed in dB, between the desired signal

amplitude to the amplitude of the peak spurious spectral component, where a spurious spectral

component is any signal present in the output spectrum that is not present at the input and may or may

not be a harmonic.

TOTAL HARMONIC DISTORTION (THD) is the ratio, expressed in dB or dBc, of the rms total of the first five

harmonic components at the output to the rms level of the input signal frequency as seen at the output.

THD is calculated as

where

•

•

(2)

THROUGHPUT TIME is the minimum time required between the start of two successive conversion. It is the

acquisition time plus the conversion and read out times. In the case of the ADC124S021, this is 16 SCLK

periods.

8

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