AD7476/AD7477/AD7478
Rev. F | Page 12 of 24
TERMINOLOGY
Integral Nonlinearity
This is the maximum deviation from a straight line passing
through the endpoints of the ADC transfer function. For the
AD7476/AD7477, 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. For the
AD7478, 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.
Differential Nonlinearity
This is the difference between the measured and the ideal 1 LSB
change between any two adjacent codes in the ADC.
Offset Error
This is the deviation of the first code transition (00 . . . 000) to
(00 . . . 001) from the ideal (such as AGND + 0.5 LSB). For the
AD7478, this is the deviation of the first code transition
(00 . . . 000) to (00 . . . 001) from the ideal (such as
AGND + 1 LSB).
Gain Error
For the AD7476/AD7477, this is the deviation of the last code
transition (111 . . . 110) to (111 . . . 111) from the ideal (such as
V
REF
– 1.5 LSB) after the offset error has been adjusted out. For
the AD7478, this is the deviation of the last code transition
(111 . . . 110) to (111 . . . 111) from the ideal (such as V
REF
– 1
LSB) after the offset error has been adjusted.
Track-and-Hold Acquisition Time
The track-and-hold amplifier returns into track mode after the
end of conversion. Track-and-hold acquisition time is the time
required for the output of the track-and-hold amplifier to reach
its final value, within ±0.5 LSB, after the end of conversion. See
the Serial Interface section for more details.
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 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 signal-to-(noise +
distortion) ratio for an ideal N-bit converter with a sine wave
input is given by
Signal-to-(Noise + Distortion) = (6.02N + 1.76) dB
Thus, for a 12-bit converter, this is 74 dB; for a 10-bit converter
it is 62 dB; and for an 8-bit converter it is 50 dB.
Total Una djusted Er ror
This is a comprehensive specification that includes gain error,
linearity error, and offset error.
Total Harmonic Distortion (THD)
Total harmonic distortion is the ratio of the rms sum of
harmonics to the fundamental. For the AD7476/
AD7477/AD7478, it is defined as:
()
1
2
6
2
5
2
4
2
3
2
2
log20dB
V
VVVVV
THD
++++
=
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
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 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, 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),
while the third-order terms include (2fa + fb), (2fa − fb),
(fa + 2fb), and (fa − 2fb).
The AD7476/AD7477/AD7478 are tested using the CCIF
standard where two input frequencies are used (fa = 498.7 kHz
and fb = 508.7 kHz). 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. 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 dB.
