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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： | 7/12頁 |
| 文件大小： | 263K |
| 代理商： | AD7821 |
AD7821
REV. A
–7–
Figure 4. Power Supply as Reference.
Unipolar Operation (0 to + 5 V)
Figure 5. External Reference.
Bipolar Operation (–2.5 V to +2.5 V)
INPUT CURRE NT
T he analog input of the AD7821 behaves somewhat differently
to conventional A/D converters. T his is due to the ADC’s
sampled data comparators, which take varying amounts of input
current depending on the cycle of the converter.
T he equivalent input circuit of the AD7821 is shown in Figure
6. When a conversion ends (e.g., falling edge of
INT
, WR-RD
mode, t
RD
> t
INT L
) all the input switches are closed and V
IN
is
connected to the comparators of the internal LS and MS ADCs.
T herefore, V
IN
is connected to 31 one-pF input capacitors
simultaneously .
Figure 6. AD7821 Equivalent Input Circuit
T he input capacitors must charge to the input voltage through
the on resistance of the analog switches (about 2 k
to 5 k
).
In addition, about 12 pF of input stray capacitance must be
charged.
T he analog input can be modeled as an equivalent RC network
as shown in Figure 7. As R
S
(source impedance) increases, the
input capacitance takes longer to charge.
T he comparators track the analog input between conversions. A
minimum delay time (t
P
) of 350 ns is required between conver-
sions to allow for voltage source settling and comparator track-
ing time. T his allows input time constants of 50 ns without
settling time problems. T ypical total input capacitance values of
55 pF allow R
S
to be 0.9 k
without lengthening t
P
to give V
IN
more time to settle.
Figure 7. RC Network Model
INPUT T RANSIE NT S
T ransients on the analog input signal caused by charging
current flowing into V
IN
will not normally degrade the ADC’s
performance. In effect, the AD7821 does not “look” at the in-
put when these transients occur. T he comparators’ inputs track
V
IN
and are not sampled until the falling edge of
WR
(WR-RD
Mode) or
RD
(RD Mode), so at least 350 ns (t
P
) is provided to
charge the ADC’s input capacitance. It is, therefore, not neces-
sary to filter out these transients with an external capacitor at
the V
IN
terminal.
INHE RE NT T RACK -AND-HOLD
A major benefit of the AD7821’s input structure is its ability to
measure a variety of high-speed signals without the help of an
external track-and-hold. Any ADC which does not have a built-
in track-and-hold, regardless of its speed, requires the analog in-
put to remain stable to at least 1/2 LSB for the duration of the
conversion to maintain full accuracy. T his requires the use of a
track-and-hold whenever the input is a high-speed signal. T he
AD7821’s sampled-data comparators, by nature of their input
switching, inherently accomplish this track-and-hold function.
Although the conversion time for the AD7821 is 660 ns
(WR-RD mode, t
WR
+ t
RD
+ t
ACC1
), the time for which V
IN
must
be stable to 1/2 LSB is much smaller. T he AD7821 tracks V
IN
between conversions only, and its value on the falling edge of
WR
or
RD
in the WR-RD or RD modes, respectively, is the
measured value.
SINUSOIDAL INPUT S
T he bandwidth of the built-in track-and-hold is 100 kHz max
(150 kHz typ, 5 V p-p). T his is limited by the analog bandwidth
of the comparators and timing skew between the comparator
switches. T his means that the analog input frequency can be up
to 100 kHz without the aid of an external track-and-hold. T he
Nyquist criterion requires that the sampling rate be at least
twice the input frequency (i.e.,
≥
2
3
100 kHz). T his requires an
ideal antialiasing filter with an infinite roll-off. T o ease the prob-
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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 |
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| 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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