Definition of Specifications–AD677
NYQUIST FREQUENCY
INTERMODULATION DISTORTION (IMD)
An implication of the Nyquist sampling theorem, the “Nyquist
frequency’’ of a converter is that input frequency which is one
half the sampling frequency of the converter.
TOTAL HARMONIC DISTORTION
Total harmonic distortion (THD) is the ratio of the rms sum of
the harmonic components to the rms value of a full-scale input
signal and is expressed in percent (%) or decibels (dB). For in-
put signals or harmonics that are above the Nyquist frequency,
the aliased components are used.
SIGNAL-TO-NOISE PLUS DISTORTION RATIO
With inputs consisting of sine waves at two frequencies, fa and
fb, any device with nonlinearities will create distortion products,
of order (m+n), at sum and difference frequencies of mfa
±
nfb,
where m, n = 0, 1, 2, 3 . . . . Intermodulation terms are those
for which m or n is not equal to zero. For example, the second
order terms are (fa + fb) and (fa – fb), and the third order terms
are (2 fa + fb), (2 fa – fb), (fa + 2 fb) and (fa – 2 fb). The IMD
products are expressed as the decibel ratio of the rms sum of the
measured input signals to the rms sum of the distortion terms.
The two signals applied to the converter are of equal amplitude,
and the peak value of their sum is –0.5 dB from full scale. The
IMD products are normalized to a 0 dB input signal.
APERTURE DELAY
Signal-to-noise plus distortion is defined to be the ratio of the
rms value of the measured input signal to the rms sum of all
other spectral components below the Nyquist frequency, includ-
ing harmonics but excluding dc.
+/– FULL-SCALE ERROR
Aperture delay is the time required after SAMPLE pin is taken
LOW for the internal sample-hold of the AD677 to open, thus
holding the value of V
IN
.
APERTURE JITTER
The last + transition (from 011 . . . 10 to 011 . . . 11) should
occur for an analog voltage 1.5 LSB below the nominal full
scale (4.99977 volts for a
±
5 V range). The full-scale error is
the deviation of the actual level of the last transition from the
ideal level.
BIPOLAR ZERO ERROR
Aperture jitter is the variation in the aperture delay from sample
to sample.
POWER SUPPLY REJECTION
Bipolar zero error is the difference between the ideal midscale
input voltage (0 V) and the actual voltage producing the mid-
scale output code.
DIFFERENTIAL NONLINEARITY (DNL)
In an ideal ADC, code transitions are one LSB apart. Differen-
tial nonlinearity is the maximum deviation from this ideal value.
It is often specified in terms of resolution for which no missing
codes are guaranteed.
INTEGRAL NONLINEARITY (INL)
DC variations in the power supply voltage will affect the overall
transfer function of the ADC, resulting in zero error and full-
scale error changes. Power supply rejection is the maximum
change in either the bipolar zero error or full-scale error value.
Additionally, there is another power supply variation to con-
sider. AC ripple on the power supplies can couple noise into the
ADC, resulting in degradation of dynamic performance. This is
displayed in Figure 15.
INPUT SETTLING TIME
The ideal transfer function for an ADC is a straight line bisect-
ing the center of each code drawn between “zero” and “full
scale.” The point used as “zero” occurs 1/2 LSB before the
most negative code transition. “Full scale” is defined as a level
1.5 LSB beyond the most positive code transition. Integral non-
linearity is the worst-case deviation of a code center average
from the straight line.
BANDWIDTH
Settling time is a function of the SHA’s ability to track fast
slewing signals. This is specified as the maximum time required
in track mode after a full-scale step input to guarantee rated
conversion accuracy.
NOISE/DC CODE UNCERTAINTY
The full-power bandwidth is that input frequency at which the
amplitude of the reconstructed fundamental is reduced by 3 dB
for a full-scale input.
Ideally, a fixed dc input should result in the same output code
for repetitive conversions. However, as a consequence of un-
avoidable circuit noise within the wideband circuits in the ADC,
there is a range of output codes which may occur for a given in-
put voltage. If you apply a dc signal to the ADC and record a
large number of conversions, the result will be a distribution of
codes. If you fit a Gaussian probability distribution to the histo-
gram, the standard deviation is approximately equivalent to the
rms input noise of the ADC.
REV. A
–7–
AD [ ANALOG DEVICES ]
