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參數(shù)資料
| 型號(hào)： | AD776 |
| 廠商： | Analog Devices, Inc. |
| 英文描述： | 16-Bit 100 kSPS Oversampling ADC(16位100kSPS過采樣A/D轉(zhuǎn)換器) |
| 中文描述： | 16位100 kSPS的采樣ADC的（16位100ksps的過采樣的A / D轉(zhuǎn)換器） |
| 文件頁數(shù)： | 5/12頁 |
| 文件大?。?/td> | 257K |
| 代理商： | AD776 |
AD776
REV. A
–5–
GE NE RAL OVE RVIE W
T he AD776 is a single supply (+5 V) ADC providing simple
analog and digital interface requirements. A minimal number
of external connections are required to achieve the specified
performance:
1. POWER
2. GROUNDS
3. CLOCK ING
4. INPUT BUFFER CIRCUIT
T hese points will be further explored in the Application Infor-
mation section.
T HE ORY OF OPE RAT ION
T he AD776 differs from traditional multibit ADCs through its
use of sigma-delta conversion architecture. A 1-bit analog-to-
T OT AL HARMONIC DIST ORT ION
T otal Harmonic Distortion (T HD) 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).
SIGNAL-T O-NOISE RAT IO (SNR)
Signal-to-Noise Ratio (SNR) 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 passband.
INT E RMODULAT ION DIST ORT ION (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 or-
der terms are (fa+fb) and (fa–fb), and the third order terms are
(2fa+fb), (2fa–fb), (fa+2fb) and (fa–2fb). T he IMD products
are expressed as the decibel ratio of the rms sum of the mea-
sured input signals to the rms sum of the distortion terms. T he
two signals applied to the converter are of equal amplitude and
the IMD products are normalized to a 0 dB input signal.
DIFFE RE NT IAL NONLINE ARIT Y (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.
INT E GRAL NONLINE ARIT Y (INL)
T he ideal transfer function for an ADC is a straight line drawn
between “zero” and “full scale.” T he point used as “zero” oc-
curs 1/2 LSB before the most negative code transition. “Full
scale” is defined as a level 1 1/2 LSB beyond the most positive
code transition. INL is the worst-case deviation of a code center
average from the straight line.
GAIN E RROR
T he last transition should ideally occur at an analog value
1.5 LSB below the nominal full scale. T he gain error is the de-
viation of the actual difference between the first and last code
transition from the ideal difference between the first and last
code transition.
digital conversion is performed at a very high rate, which redis-
tributes quantization noise to beyond the frequency band of
interest (see Figure 1). T he frequency band of interest is denoted
by f
C
, and f
S
is the sample frequency; f
S
/2 is the expanded noise
spectrum resulting from oversampling. T he total noise energy
remains constant, but by spreading it over a wider spectrum, the
amount in the frequency band of interest is reduced. Noise
shaping, performed by the modulator, attenuates noise in the
signal passband and pushes out the noise energy into the higher
frequency range (Figure 2). T he oversampled signal is pre-
sented to the digital filter circuitry for:
– sophisticated averaging (filtering).
– removing high frequency noise (quantization noise removal).
– reducing sampling rate (decimation).
MIDSCALE E RROR
Midscale error is the difference between the ideal midscale ana-
log input voltage and the actual voltage producing the midscale
output code.
PASSBAND
T he passband is the region of the frequency spectrum unaf-
fected by the attenuation of the decimation filter. In the case of
the AD776, the passband is a function of the CLK IN frequency
(see T able I).
PASSBAND RIPPLE
Passband ripple is defined as the variation in the amplitude
response of the converter for input signal frequencies within
the passband.
ST OPBAND
T he stopband is the region of the frequency spectrum in which
the amplitude response is fully attenuated by the digital filter. In
the case of the AD776, the stopband is a function of the CLK IN
frequency (see T able I).
ST OPBAND AT T E NUAT ION
Stopband attenuation is defined to be the amount by which
spectral components in the stopband are attenuated by the digi-
tal filter relative to the full-scale input range of the converter.
POWE R SUPPLY RE JE CT ION
DC variations in the power supply voltage will affect the mid-
scale transition point, resulting in offset error. Power supply
rejection is the maximum change in the midscale transition
point due to a change in power-supply voltage from the nominal
value. Additionally, there is another power supply variation to
consider. AC ripple on the power supplies can couple noise into
the ADC, resulting in degradation of dynamic performance.
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