AD7484

Rev. C | Page 11 of 20

TERMINOLOGY

Integral Nonlinearity

The integral nonlinearity 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 1/2 LSB below the first code transition, and full scale, a

point 1/2 LSB above the last code transition.

Differential Nonlinearity

The differential nonlinearity is the difference between the

measured and ideal 1 LSB change between any two adjacent

codes in the ADC.

Offset Error

The offset error is the deviation of the first code transition

(00…000) to (00…001) from the ideal, that is, AGND + 0.5 LSB.

Gain Error

The gain error is the deviation of the last code transition

after the offset error has been adjusted out.

Track-and-Hold Acquisition Time

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 ±1/2 LSB, after the end of conversion (the point at which

the track-and-hold returns to track mode).

Signal-to-Noise + Distortion (SINAD) Ratio

The SINAD ratio is the measured ratio of signal-to-noise +

distortion at the output of the ADC. The signal is the rms ampli-

tude of the fundamental. Noise is the sum of all nonfundamental

ratio is dependent on the number of quantization levels in the

digitization process; the more levels, the smaller the quantization

noise. The theoretical SINAD ratio for an ideal N-bit converter

with a sine wave input is given by

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

Therefore, this is 86.04 dB for a 14-bit converter.

Total Harmonic Distortion (THD)

The THD is the ratio of the rms sum of the harmonics to the

fundamental. It is defined as

( )

1

2

6

2

5

2

4

2

3

2

2

V

V

V

V

V

V

THD

+

+

+

+

=

log

20

dB

sixth harmonics.

Peak Harmonic or Spurious Noise

The peak harmonic or spurious noise is the ratio of the rms

value of the next largest component in the ADC output spectrum

The value of this specification is usually 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.

Intermodulation Distortion

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

fb, any active device with nonlinearities creates 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 is equal to zero. For

example, the second order terms include (fa + fb) and (fa − fb),

whereas the third order terms include (2fa + fb), (2fa − fb), (fa +

2fb), and (fa − 2fb).

The AD7484 is tested using the CCIF standard where two input

frequencies near the top end of the input bandwidth are used.

In this case, the second order terms are usually distanced in

frequency from the original sine waves, whereas the third order

terms are usually at a frequency close to the input frequencies.

As a result, the second order and third order terms are specified

separately. The calculation of the intermodulation distortion is

as 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.