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ADC08831 Просмотр технического описания (PDF) - National ->Texas Instruments

Номер в каталогеADC08831 National-Semiconductor
National ->Texas Instruments National-Semiconductor
Компоненты Описание8-Bit Serial I/O CMOS A/D Converters with Multiplexer and Sample/Hold Function
ADC08831 Datasheet PDF : 24 Pages
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Functional Description (Continued)
The VREFIN (or VCC) voltage is then adjusted to provide a
code change from FEHEX to FFHEX. This completes the ad-
justment procedure.
Dynamic performance specifications are often useful in ap-
plications requiring waveform sampling and digitization.
Typically, a memory buffer is used to capture a stream of
consecutive digital outputs for post processing. Capturing a
number of samples that is a power of 2 (ie, 1024, 2048,
4096) allows the Fast Fourier Transform (FFT) to be used to
digitally analyze the frequency components of the signal.
Depending on the application, further digital filtering, win-
dowing, or processing can be applied.
7.1 Sampling Rate
The Sampling Rate, sometimes referred to as the Through-
put Rate, is the time between repetitive samples by an
Analog-to-Digital Converter. The sampling rate includes the
conversion time, as well as other factors such a MUX setup
time, acquisition time, and interfacing time delays. Typically,
the sampling rate is specified in the number of samples
taken per second, at the maximum Analog-to-Digital Con-
verter clock frequency.
Signals with frequencies exceeding the Nyquist frequency
(1/2 the sampling rate), will be aliased into frequencies be-
low the Nyquist frequency. To prevent signal degradation,
sample at twice (or more) than the input signal and/or use of
a low pass (anti-aliasing) filter on the front-end. Sampling at
a much higher rate than the input signal will reduce the re-
quirements of the anti-aliasing filter.
Some applications require under-sampling the input signal.
In this case, one expects the fundamental to be aliased into
the frequency range below the Nyquist frequency. In order to
be assured the frequency response accurately represents a
harmonic of the fundamental, a band-pass filter should be
used over the input range of interest.
7.2 Signal-to-Noise Ratio
Signal-to-Noise Ratio (SNR) is the ratio of RMS magnitude
of the fundamental to the RMS sum of all the
non-fundamental signal, excluding the harmonics, up to 1/2
of the sampling frequency (Nyquist).
7.3 Total Harmonic Distortion
Total Harmonic distortion is the ratio of the RMS sum of the
amplitude of the harmonics to the fundamental input fre-
THD = 20 log [(V22 + V32+ V42+ V52+ V62) 1/2/V1]
Where V1 is the RMS amplitude of the fundamental and
V2,V3, V4, V5, V6 are the RMS amplitudes of the individual
harmonics. In theory, all harmonics are included in THD cal-
culations, but in practice only about the first 6 make signifi-
cant contributions and require measurement.
For under-sampling applications, the input signal should be
band pass filtered (BPF) to prevent out of band signals, or
their harmonics, to appear in the spectral response.
The DC Linearity transfer function of an Analog-to-Digital
Converter tends to influence the dominant harmonics. A
parabolic Linearity curve would tend to create 2nd (and even)
order harmonics, while an S-curve would tend to create 3rd
(or odd) order harmonics. The magnitude of an DC linearity
error correlates to the magnitude of the harmonics.
7.4 Signal-to-Noise and Distortion
Signal-to-Noise And Distortion ratio (SINAD) is the ratio of
RMS magnitude of the fundamental to the RMS sum of all
the non-fundamental signals, including the noise and har-
monics, up to 1/2 of the sampling frequency (Nyquist), ex-
cluding DC.
SINAD is also dependent on the number of quantization lev-
els in the A/D Converter used in the waveform sampling pro-
cess. The more quantization levels, the smaller the quantiza-
tion noise and theoretical noise performance. The theoretical
SINAD for a N-Bit Analog-to-Digital Converter is given by:
SINAD = (6.02 N + 1.76) dB
Thus, for an 8-bit converter, the ideal SINAD = 49.92 dB
7.5 Effective Number of Bits
Effective Number Of Bits (ENOB) is another specification to
quantify dynamic performance. The equation for ENOB is
given by:
ENOB = [(SINAD - 1.76)] / 6.02]
The Effective Number Of Bits portrays the cumulative effect
of several errors, including quantization, non-linearities,
noise, and distortion.
7.6 Spurious Free Dynamic Range
Spurious Free Dynamic Range (SFDR) is the ratio of the sig-
nal amplitude to the amplitude of the highest harmonic or
spurious noise component. If the amplitude is at full scale,
the specification is simply the reciprocal of the peak har-
monic or spurious noise.
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