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| 型号： | AD9251BCPZ-20 |
| 厂商： | Analog Devices Inc |
| 文件页数： | 14/36页 |
| 文件大小： | 0K |
| 描述： | IC ADC 14BIT 20MSPS 64LFCSP |
| 标准包装： | 1 |
| 位数： | 14 |
| 采样率（每秒）： | 20M |
| 数据接口： | 串行，SPI? |
| 转换器数目： | 2 |
| 功率耗散（最大）： | 77mW |
| 电压电源： | 模拟和数字 |
| 工作温度： | -40°C ~ 85°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 64-VFQFN 裸露焊盘，CSP |
| 供应商设备封装： | 64-LFCSP-VQ（9x9） |
| 包装： | 托盘 |
| 输入数目和类型： | 4 个单端，单极；2 个差分，单极 |
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AD9251
Rev. A | Page 21 of 36
Input Common Mode
The analog inputs of the AD9251 are not internally dc-biased.
Therefore, in ac-coupled applications, the user must provide a
dc bias externally. Setting the device so that VCM = AVDD/2 is
recommended for optimum performance, but the device can
function over a wider range with reasonable performance, as
An on-board, common-mode voltage reference is included in
the design and is available from the VCM pin. The VCM pin
must be decoupled to ground by a 0.1 μF capacitor, as described
in the Applications Information section.
100
90
80
70
60
50
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
1.3
INPUT COMMON-MODE VOLTAGE (V)
S
N
R
/SF
D
R
(
d
B
F
S/
d
B
c
)
07
93
8-
0
49
SFDR (dBc)
SNR (dBFS)
Figure 39. SNR/SFDR vs. Input Common-Mode Voltage,
fIN = 32.1 MHz, fS = 80 MSPS
100
90
80
70
60
50
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
1.3
INPUT COMMON-MODE VOLTAGE (V)
S
N
R/
S
F
DR
(
d
B
F
S
/d
B
c
)
07
93
8-
0
50
SFDR (dBc)
SNR (dBFS)
Figure 40. SNR/SFDR vs. Input Common-Mode Voltage,
fIN = 10.3 MHz, fS = 20 MSPS
Differential Input Configurations
Optimum performance is achieved while driving the AD9251 in a
differential input configuration. For baseband applications, the
excellent performance and a flexible interface to the ADC.
The output common-mode voltage of the
ADA4938-2 is easily
set with the VCM pin of the AD9251 (see Figure 41), and the
driver can be configured in a Sallen-Key filter topology to
provide band limiting of the input signal.
AVDD
VIN
76.8
120
0.1F
33
33
10pF
200
200
90
ADA4938
ADC
VIN–x
VIN+x
VCM
07
93
8-
0
07
Figure 41. Differential Input Configuration Using the ADA4938-2
For baseband applications below ~10 MHz where SNR is a key
parameter, differential transformer-coupling is the recommended
input configuration. An example is shown in Figure 42. To bias
the analog input, the VCM voltage can be connected to the
center tap of the secondary winding of the transformer.
2V p-p
49.9
0.1F
R
C
0
793
8-
0
08
ADC
VCM
VIN+x
VIN–x
Figure 42. Differential Transformer-Coupled Configuration
The signal characteristics must be considered when selecting
a transformer. Most RF transformers saturate at frequencies
below a few megahertz (MHz). Excessive signal power can also
cause core saturation, which leads to distortion.
At input frequencies in the second Nyquist zone and above, the
noise performance of most amplifiers is not adequate to achieve
the true SNR performance of the AD9251. For applications above
~10 MHz where SNR is a key parameter, differential double balun
coupling is the recommended input configuration (see Figure 44).
An alternative to using a transformer-coupled input at frequencies
in the second Nyquist zone is to use the AD8352 differential driver.
An example is shown in Figure 45. See the AD8352 data sheet
for more information.
In any configuration, the value of Shunt Capacitor C is dependent
on the input frequency and source impedance and may need to
be reduced or removed. Table 9 displays the suggested values to set
the RC network. However, these values are dependent on the
input signal and should be used only as a starting guide.
Table 9. Example RC Network
Frequency Range (MHz)
R Series
(Ω Each)
C Differential (pF)
0 to 70
33
22
70 to 200
125
Open
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