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參數資料
| 型號： | AD7821 |
| 廠商： | Analog Devices, Inc. |
| 英文描述： | LC2MOS High Speed, μP-Compatible 8-Bit ADC with Track/Hold Function(帶跟蹤/保持功能,LC2MOSμP兼容8位高速A/D轉換器) |
| 中文描述： | LC2MOS高速，微處理器兼容的8位ADC的采樣/保持功能（帶跟蹤/保持功能，LC2MOSμP兼容8位高速的A / D轉換器） |
| 文件頁數： | 8/12頁 |
| 文件大小： | 263K |
| 代理商： | AD7821 |
AD7821
REV. A
–8–
lem of antialiasing filter design, the sampling rate is usually set
much greater than the Nyquist criterion. T he maximum sam-
pling rate (f
MAX
) for the AD7821 in the WR-RD mode,
(t
RD
< t
INT L
) can be calculated as follows:
f
MAX
=
1
t
WR
+
t
RD
+
t
RI
+
t
P
f
MAX
=
1
0.25
E
6
+
0.25
E
6
+
0.15
E
6
+
0.35
E
6
t
WR
= Write Pulse Width
t
RD
= Delay Time between
WR
and
RD
Pulses
t
RI
=
RD
to
INT
Delay
t
P
= Delay Time between Conversions
T his permits a maximum sampling rate for the AD7821 of
1 MHz, which is much greater than the Nyquist criterion for
sampling a 100 kHz analog input signal.
DIGIT AL SIGNAL PROCE SSING APPLICAT IONS
In Digital Signal Processing (DSP) application areas like voice
recognition, echo cancellation and adaptive filtering, the dy-
namic characteristics (Signal-to-Noise Ratio, Harmonic Distor-
tion, Intermodulation Distortion) of an ADC are critical. Since
the AD7821 is a very fast ADC with a built-in track-and-hold
function, it is specified dynamically as well as with standard dc
specifications (T otal Unadjusted Error, etc.).
SIGNAL-T O-NOISE RAT IO AND DIST ORT ION
T he dynamic performance of the AD7821 is evaluated by apply-
ing a very low distortion sine wave signal to the analog input
(V
IN
) which is then sampled at a 512 kHz sampling rate. A Fast
Fourier T ransform (FFT ) plot is then generated from which
Signal-to-Noise Ratio (SNR) and harmonic distortion data are
obtained.
Figure 8 shows a 2048 point FFT plot of the AD7821 with an
input signal of 100.25 kHz. T he SNR is 49.1 dB. It should be
noted that the harmonics are taken into account when calculat-
ing the SNR. T he theoretical relationship between SNR and
resolution (N) is expressed by the following equation:
SNR = (
6.02
N +
1.76
) dB . . . . . . . . . . . . . . . . . . . . .
(1)
Figure 8. AD7821 FFT Plot
E FFE CT IVE NUMBE R OF BIT S
By working backwards from Equation (1) it is possible to get a
measure of ADC performance expressed in effective number of
bits (N). A plot of the effective number of bits versus input fre-
quency is given in the T ypical Performance Characteristics sec-
tion. T he effective number of bits typically falls between 7.7 and
7.9, corresponding to SNR figures of 48.1 and 49.7 dB.
INT E RMODULAT ION DIST ORT ION
For intermodulation distortion (IMD), an FFT plot consisting
of very low distortion sine waves at two frequencies is generated
by sampling an analog input applied to the ADC. Figure 9
shows a 2048 point plot for IMD.
Figure 9. FFT Plot for IMD
HIST OGRAM PLOT
When a sine wave of specified frequency is applied to the V
IN
in-
put of the AD7821 and several thousand samples are taken, it is
possible to plot a histogram showing the frequency of occur-
rence of each of the 256 ADC codes. A perfect ADC produces a
probability density function described by the equation:
P
(
V
)
=
1
π
(
A
2
V
2
)
1/2
where
A
is the peak amplitude of the sine wave and
P
(
V
) is the
probability of occurrence at a voltage V.
If a particular step is wider than the ideal 1 LSB width, then the
code associated with that step will accumulate more counts than
for the code for an ideal step. Likewise, a step narrower than the
ideal width will have fewer counts. Missing codes are easily seen
because a missing code means zero counts for a particular code.
T he absence of large spikes in the plot indicates small differen-
tial nonlinearity.
Figure 10 shows a histogram plot for the AD7821, which corre-
sponds very well with the ideal shape. T he plot indicates very
small differential nonlinearity and no missing codes for an input
frequency of 100.25 kHz.
Figure 10. AD7821 Histogram Plot
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相關代理商/技術參數 |
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| AD7821BQ | 功能描述:IC ADC 8BIT HS TRACK/HOLD 20CDIP RoHS:否 類別:集成電路 (IC) >> 數據采集 - 模數轉換器 系列:- 標準包裝:1 系列:- 位數:14 采樣率（每秒）:83k 數據接口:串行，并聯 轉換器數目:1 功率耗散（最大）:95mW 電壓電源:雙 ± 工作溫度:0°C ~ 70°C 安裝類型:通孔 封裝/外殼:28-DIP（0.600"，15.24mm） 供應商設備封裝:28-PDIP 包裝:管件 輸入數目和類型:1 個單端，雙極 |
| AD7821KN | 功能描述:IC ADC 8BIT LC2MOS T/H HS 20-DIP RoHS:否 類別:集成電路 (IC) >> 數據采集 - 模數轉換器 系列:- 標準包裝:1,000 系列:- 位數:12 采樣率（每秒）:300k 數據接口:并聯 轉換器數目:1 功率耗散（最大）:75mW 電壓電源:單電源 工作溫度:0°C ~ 70°C 安裝類型:表面貼裝 封裝/外殼:24-SOIC（0.295"，7.50mm 寬） 供應商設備封裝:24-SOIC 包裝:帶卷 (TR) 輸入數目和類型:1 個單端，單極；1 個單端，雙極 |
| AD7821KNZ | 功能描述:IC ADC 8BIT LC2MOS T/H HS 20-DIP RoHS:是 類別:集成電路 (IC) >> 數據采集 - 模數轉換器 系列:- 標準包裝:1,000 系列:- 位數:12 采樣率（每秒）:300k 數據接口:并聯 轉換器數目:1 功率耗散（最大）:75mW 電壓電源:單電源 工作溫度:0°C ~ 70°C 安裝類型:表面貼裝 封裝/外殼:24-SOIC（0.295"，7.50mm 寬） 供應商設備封裝:24-SOIC 包裝:帶卷 (TR) 輸入數目和類型:1 個單端，單極；1 個單端，雙極 |
| AD7821KNZ | 制造商:Analog Devices 功能描述:IC 8-BIT ADC |
| AD7821KP | 制造商:AD 制造商全稱:Analog Devices 功能描述:LC2MOS High Speed, mP-Compatible 8-Bit ADC with Track/Hold Function |
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