参数资料
型号: AD9251BCPZ-20
厂商: Analog Devices Inc
文件页数: 17/36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
AD9251
Rev. A | Page 24 of 36
External Reference Operation
The use of an external reference may be necessary to enhance
the gain accuracy of the ADC or improve thermal drift charac-
teristics. Figure 48 shows the typical drift characteristics of the
internal reference in 1.0 V mode.
4
3
2
1
0
–1
–2
–3
–4
–5
–6
–40
–20
0
20
40
60
80
TEMPERATURE (°C)
V
RE
F
E
RRO
R
(m
V
)
07
938
-05
2
VREF ERROR (mV)
Figure 48. Typical VREF Drift
When the SENSE pin is tied to AVDD, the internal reference is
disabled, allowing the use of an external reference. An internal
reference buffer loads the external reference with an equivalent
7.5 kΩ load (see Figure 37). The internal buffer generates the
positive and negative full-scale references for the ADC core.
Therefore, the external reference must be limited to a maximum
of 1.0 V.
CLOCK INPUT CONSIDERATIONS
For optimum performance, clock the AD9251 sample clock
inputs, CLK+ and CLK, with a differential signal. The signal
is typically ac-coupled into the CLK+ and CLK pins via a
transformer or capacitors. These pins are biased internally
(see Figure 49) and require no external bias.
0.9V
AVDD
2pF
CLK–
CLK+
0
79
38-
0
16
Figure 49. Equivalent Clock Input Circuit
Clock Input Options
The AD9251 has a very flexible clock input structure. The clock
input can be a CMOS, LVDS, LVPECL, or sine wave signal.
Regardless of the type of signal being used, clock source jitter is
of the most concern, as described in the Jitter Considerations
section.
Figure 50 and Figure 51 show two preferred methods for clock-
ing the AD9251 (at clock rates up to 625 MHz). A low jitter clock
source is converted from a single-ended signal to a differential
signal using either an RF transformer or an RF balun.
The RF balun configuration is recommended for clock frequencies
between 125 MHz and 625 MHz, and the RF transformer is recom-
mended for clock frequencies from 10 MHz to 200 MHz. The
back-to-back Schottky diodes across the transformer/balun
secondary limit clock excursions into the AD9251 to
approximately 0.8 V p-p differential.
This limit helps prevent the large voltage swings of the clock
from feeding through to other portions of the AD9251 while
preserving the fast rise and fall times of the signal that are critical
to a low jitter performance.
0.1F
SCHOTTKY
DIODES:
HSMS2822
CLOCK
INPUT
50
100
CLK–
CLK+
ADC
Mini-Circuits
ADT1-1WT, 1:1 Z
XFMR
07
93
8-
01
7
Figure 50. Transformer-Coupled Differential Clock (Up to 200 MHz)
0.1F
1nF
CLOCK
INPUT
1nF
50
CLK–
CLK+
SCHOTTKY
DIODES:
HSMS2822
07
93
8-
01
8
ADC
Figure 51. Balun-Coupled Differential Clock (Up to 625 MHz)
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