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參數(shù)資料
| 型號： | ADS821E |
| 英文描述： | 10-Bit, 40MHz Sampling ANALOG-TO-DIGITAL CONVERTER |
| 中文描述： | 10位，40MHz的采樣模擬數(shù)字轉(zhuǎn)換器 |
| 文件頁數(shù)： | 11/18頁 |
| 文件大?。?/td> | 384K |
| 代理商： | ADS821E |
www.ti.com
ADS821
SBAS040B
11
APPLICATIONS
DRIVING THE ADS821
The ADS821 has a differential input with a common-mode of
+2.25V. For AC-coupled applications, the simplest way to
create this differential input is to drive the primary winding of
a transformer with a single-ended input. A differential output
is created on the secondary if the center tap is tied to the
common-mode (CM) voltage of +2.25V, as per Figure 5. This
transformer-coupled input arrangement provides good high-
frequency AC performance. It is important to select a trans-
former that gives low distortion and does not exhibit core
saturation at full-scale voltage levels. Since the transformer
does not appreciably load the ladder, there is no need to
buffer the CM output in this instance. In general, it is
advisable to keep the current draw from the CM output pin
below 0.5
μ
A to avoid nonlinearity in the internal reference
ladder. A FET input operational amplifier such as the OPA130
can provide a buffered reference for driving external circuitry.
The analog IN and
IN
inputs should be bypassed with 22pF
capacitors to minimize track-and-hold glitches and to im-
prove high-input frequency performance.
Figure 6 shows an AC-coupled single-ended input interface
circuit using the low-cost, current feedback OPA694 as the
active gain stage. When testing this configuration in gains of
+4, +5.8, and +8.2, it was noted that reducing the feedback
resistor of the OPA694 from the typical 402
to 360
resulted in a wider bandwidth, thus improving distortion at
higher gains. The gain resistor was scaled to 120
, 75
, and
50
for each of the three gain settings. The two 330
resistors set the RC time constant and the values can be
varied, although higher values will have the effect of moving
the corner frequency of the created high-pass filter down. In
Figure 6, the
–
3dB point is set at 4.2kHz.
Figure 7 illustrates another possible low-cost interface circuit
that utilizes resistors and capacitors in place of a transformer.
Depending on the signal bandwidth, the component values
should be carefully selected in order to maintain the perfor-
mance outlined in the data sheet. The input capacitors, C
IN
,
and the input resistors, R
IN
, create a high-pass filter with the
lower corner frequency at f
C
= 1/(2
π
R
IN
C
IN
). The corner
frequency can be reduced by either increasing the value of
R
IN
or C
IN
. If the circuit operates with a 50
or 75
imped-
ance level, the resistors are fixed and only the value of the
capacitor can be increased. Usually AC-coupling capacitors
are electrolytic or tantalum capacitors with values of 1mF or
higher. It should be noted that these large capacitors become
inductive with increased input frequency, which could lead to
signal amplitude errors or oscillation. To maintain a low AC-
coupling impedance throughout the signal band, a small
value (e.g. 1
μ
F) ceramic capacitor could be added in parallel
with the polarized capacitor.
Capacitors C
SH1
and C
SH2
are used to minimize current
glitches resulting from the switching in the input track-and-
hold stage and to improve signal-to-noise performance. These
capacitors can also be used to establish a low-pass filter and
effectively reduce the noise bandwidth. In order to create a
real pole, resistors R
SER1
and R
SER2
were added in series with
each input. The cut off frequency of the filter is determined by
f
C
= 1/(2
π
R
SER
(C
SH
+ C
ADC
)) where R
SER
is the resistor in
series with the input, C
SH
is the external capacitor from the
input to ground, and C
ADC
is the internal input capacitance of
the A/D converter (typically 4pF).
Resistors R
1
and R
2
are used to derive the necessary com-
mon-mode voltage from the buffered top and bottom refer-
ences.
The total load of the resistor string should be selected
FIGURE 5. AC-Coupled, Single-Ended to Differential Drive
Circuit Using a Transformer.
FIGURE 6. Low-Cost, AC-Coupled, Single-Ended Input Circuit.
Mini-Circuits
TT1-6-KK81
or Equivalent
22
26
27
CM
IN
IN
ADS821
AC Input
Signal
22pF
22pF
0.1
μ
F
ADS821
A
1
V
IN
R
G
OPA694
0.1 || 2.2
0.1 || 2.2
+5V
–
5V
I/O
360
330
330
IN
27
26
22
CM
+2.25V
IN
22pF
49.9
0.1
μ
F
0.1
μ
F
0.1
μ
F
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相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| ADS821E/1K | 制造商:BB 制造商全稱:BB 功能描述:10-Bit, 40MHz Sampling ANALOG-TO-DIGITAL CONVERTER |
| ADS821U | 功能描述:模數(shù)轉(zhuǎn)換器 - ADC 10-Bit 40 MSPS SE/ Diff Input RoHS:否 制造商:Texas Instruments 通道數(shù)量:2 結(jié)構(gòu):Sigma-Delta 轉(zhuǎn)換速率:125 SPs to 8 KSPs 分辨率:24 bit 輸入類型:Differential 信噪比:107 dB 接口類型:SPI 工作電源電壓:1.7 V to 3.6 V, 2.7 V to 5.25 V 最大工作溫度:+ 85 C 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:VQFN-32 |
| ADS821U | 制造商:Texas Instruments 功能描述:10 bit sampling analog/digital converter |
| ADS821U | 制造商:Texas Instruments 功能描述:IC 10BIT ADC 40SPS 28-SOIC |
| ADS821U-1 | 制造商:Rochester Electronics LLC 功能描述:- Bulk |
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