Data Sheet AD7475/AD7495
Rev. C | Page 11 of 24
TERMINOLOGY
Integral Nonlinearity
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 ½ LSB below the
first code transition, and full scale, a point ½ 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 + 0.5 LSB.
Gain Error
This is the deviation of the last code transition (111. ..110) to
(111...111) from the ideal (that is, V
REF
− 1.5 LSB) after the
offset error has been adjusted out.
Track-and-Hold Acquisition Time
The track-and-hold amplifier returns into track mode on the
13
th
SCLK rising edge (see the Serial Interface section). The
track-and-hold acquisition time is the minimum time required
for the track-and-hold amplifier to remain in track mode for its
output to reach and settle to within 0.5 LSB of the applied input
signal, given a step change to the input signal.
Signal-to-Noise and Distortion Ratio (SINAD)
The measured ratio of signal-to-noise and distortion at the
output of the analog-to-digital converter (ADC). The signal is
the rms amplitude of the fundamental. Noise is the sum of all
nonfundamental signals up to half the sampling frequency
(f
S
/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 SINAD ratio
for an ideal N-bit converter with a sine wave input is given by
( ) ( )
dB76.102.6 +=+ NDistortionNoisetoSignal
For a 12-bit converter, the SINAD is 74 dB.
Total Harmonic Distortion (THD)
The ratio of the rms sum of harmonics to the fundamental. For
the AD7475/AD7495, THD is defined as
( )
1
6
54
32
V
VVVVV
THD
22222
log20dB
++++
=
where V
1
is the rms amplitude of the fundamental and V
2
, V
3
,
V
4
, V
5
, and V
6
are the rms amplitudes of the second through the
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 f
S
/2 and 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, 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, n = 0, 1,
2, 3, etc. 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), while the third-order terms
include (2fa + fb), (2fa − fb), (fa + 2fb), and (fa − 2fb).
The AD7475/AD7495 are 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 while the third order
terms are usually at a frequency close to the input frequencies.
As a result, the second- and third-order terms are specified
separately. Like THD, intermodulation distortion is calculated
as the rms sum of the individual distortion products to the rms
amplitude of the sum of the fundamentals, expressed in dBs.
