AD7910/AD7920
Rev. C | Page 12 of 24
TERMINOLOGY
Integral Nonlinearity
The maximum deviation from a straight line passing through
the endpoints of the ADC transfer function. For the AD7920
and AD7910, 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
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, GND + 1 LSB.
Gain Error
The deviation of the last code transition (111 . . . 110) to
(111 . . . 111) from the ideal, that is, V
REF
− 1 LSB after the offset
error has been adjusted out.
Track-and-Hold Acquisition Time
The track-and-hold amplifier returns to track mode at 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
The measured ratio of signal-to-(noise + distortion) at the output
of the A/D converter. 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 digiti-
zation 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, and for a 10-bit
converter this is 62 dB.
Tot a l Unadjuste d Error
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. It is defined as:
1
2
6
2
5
2
4
2
3
2
2
log20)(
V
VVVVV
dBTHD
++++
=
where:
V
1
is the rms amplitude of the fundamental.
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 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 whose
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 are 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 AD7910/AD7920 are tested using the CCIF standard, where
two input frequencies are used (see fa and fb in the
Specifications
page). 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, the ratio of the rms sum of the individual distortion
products to the rms amplitude of the sum of the fundamentals,
expressed in dB.
