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AD679BJ View Datasheet(PDF) - Analog Devices

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AD679BJ Datasheet PDF : 12 Pages
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AD679
NYQUIST FREQUENCY
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.
SIGNAL-TO-NOISE AND DISTORTION (S/N+D) RATIO
S/N+D is the ratio of the rms value of the measured input signal
to the rms sum of all other spectral components below the
Nyquist frequency, including harmonics but excluding dc.
TOTAL HARMONIC DISTORTION (THD)
THD is the ratio of the rms sum of the first six harmonic com-
ponents to the rms value of a full-scale input signal and is ex-
pressed as a percentage or in decibels. For input signals or
harmonics that are above the Nyquist frequency, the aliased
component is used.
PEAK SPURIOUS OR PEAK HARMONIC COMPONENT
The peak spurious or peak harmonic component is the largest
spectral component excluding the input signal and dc. This
value is expressed in decibels relative to the rms value of a full-
scale input signal.
INTERMODULATION DISTORTION (IMD)
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 or-
der 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 distor-
tion 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 (9.44 V p-p). The IMD products are normalized to a 0-dB
input signal.
BANDWIDTH
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.
The full-linear bandwidth is the input frequency at which the
slew rate limit of the sample-hold-amplifier (SHA) is reached.
At this point, the amplitude of the reconstructed fundamental
has degraded by less than –0.1 dB. Beyond this frequency, dis-
tortion of the sampled input signal increases significantly.
The AD679 has been designed to optimize input bandwidth, al-
lowing it to undersample input signals with frequencies signifi-
cantly above the converter’s Nyquist frequency.
APERTURE DELAY
Aperture delay is a measure of the SHA’s performance and is
measured from the falling edge of Start Convert (SC) to when
the input signal is held for conversion. In synchronous mode,
Chip Select (CS) should be LOW before SC to minimize aper-
ture delay.
APERTURE JITTER
Aperture jitter is the variation in aperture delay for successive
samples and is manifested as noise on the input to the A/D.
INPUT SETTLING TIME
Settling time is a function of the SHA’s ability to track fast slew-
ing signals. This is specified as the maximum time required in
track mode after a full-scale step input to guarantee rated con-
version accuracy.
DIFFERENTIAL NONLINEARITY (DNL)
In an ideal ADC, code transitions are 1 LSB apart. Differential
linearity is the deviation from this ideal value. It is often speci-
fied in terms of resolution for which no missing codes (NMC)
are guaranteed.
INTEGRAL NONLINEARITY (INL)
The ideal transfer function for a linear ADC is a straight line
drawn between “zero” and “full scale.” The point used as
“zero” occurs 1/2 LSB before the first code transition. “Full
scale” is defined as a level 1 1/2 LSB beyond the last code tran-
sition. Integral linearity error is the worst case deviation of a
code from the straight line. The deviation of each code is mea-
sured from the middle of that code.
Note that the linearity error is not user adjustable.
POWER SUPPLY REJECTION
Variations in power supply will affect the full-scale transition,
but not the converter’s linearity. Power Supply Rejection is the
maximum change in the full-scale transition point due to a
change in power supply voltage from the nominal value.
TEMPERATURE DRIFT
This is the maximum change in the parameter from the initial
value (@ +25°C) to the value at TMIN or TMAX.
UNIPOLAR ZERO ERROR
In unipolar mode, the first transition should occur at a level
1/2 LSB above analog ground. Unipolar zero error is the devia-
tion of the actual transition from that point. This error can be
adjusted as discussed in the Input Connections and Calibration
section.
BIPOLAR ZERO ERROR
In the bipolar mode, the major carry transition (11 1111 1111
1111 to 00 0000 0000 0000 ) should occur at an analog value
1/2 LSB below analog ground. Bipolar zero error is the devia-
tion of the actual transition from that point. This error can be
adjusted as discussed in the Input Connections and Calibration
section.
GAIN ERROR
The last transition should occur at an analog value 1 1/2 LSB
below the nominal full scale (9.9991 volts for a 0 V–10 V range,
4.9991 volts for a ± 5 V range). The gain error is the deviation of
the actual level at the last transition from the ideal level with the
zero error trimmed out. This error can be adjusted as shown in
the Input Connections and Calibration section.
REV. C
–7–
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