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| 型号： | AD9273BSVZ-25 |
| 厂商： | Analog Devices Inc |
| 文件页数： | 16/48页 |
| 文件大小： | 0K |
| 描述： | IC ADC OCT 12BIT 25MSPS 100-TQFP |
| 标准包装： | 1 |
| 类型： | AAF，ADC，交叉点开关，LNA，VGA |
| 分辨率（位）： | 12 b |
| 采样率（每秒）： | 25M |
| 数据接口： | 串行，SPI? |
| 电压电源： | 模拟和数字 |
| 电源电压： | 1.8V，3V |
| 工作温度： | -40°C ~ 85°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 100-TQFP 裸露焊盘 |
| 供应商设备封装： | 100-TQFP-EP（14x14） |
| 包装： | 托盘 |
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AD9273
Rev. B | Page 23 of 48
Active Impedance Matching
The LNA consists of a single-ended voltage gain amplifier with
differential outputs and the negative output available externally.
For example, with a fixed gain of 8× (17.9 dB), an active input
termination is synthesized by connecting a feedback resistor
between the negative output pin, LO-x, and the positive input pin,
LI-x. This well-known technique is used for interfacing multiple
probe impedances to a single system. The input resistance is
shown in Equation 1.
)
2
1
(
A
R
FB
IN
+
=
(1)
where A/2 is the single-ended gain or the gain from the LI-x
inputs to the LO-x outputs, and RFB is the resulting impedance
Because the amplifier has a gain of 8× from its input to its
differential output, it is important to note that the gain A/2 is
the gain from Pin LI-x to Pin LO-x, and it is 6 dB less than the
gain of the amplifier, or 12.1 dB (4×). The input resistance is
reduced by an internal bias resistor of 15 kΩ in parallel with the
source resistance connected to Pin LI-x while Pin LG-x is ac
grounded. Equation 2 can be used to calculate the needed RFB
for a desired RIN, even for higher values of RIN.
Ω
+
=
k
15
||
)
3
1
(
FB
IN
R
(2)
For example, to set RIN to 200 Ω, the value of RFB must be
1000 Ω. If the simplified equation (Equation 2) is used to
calculate RIN, the value is 188 Ω, resulting in a gain error less
than 0.6 dB. Some factors, such as the presence of a dynamic
source resistance, might influence the absolute gain accuracy
more significantly. At higher frequencies, the input capacitance
of the LNA needs to be considered. The user must determine
the level of matching accuracy and adjust RFB accordingly.
The bandwidth (BW) of the LNA is greater than 100 MHz.
Ultimately, the BW of the LNA limits the accuracy of the
synthesized RIN. For RIN = RS up to about 200 Ω, the best match
is between 100 kHz and 10 MHz, where the lower frequency
limit is determined by the size of the ac-coupling capacitors,
and the upper limit is determined by the LNA BW. Furthermore,
the input capacitance and RS limit the BW at higher frequencies.
10
100
1k
100k
1M
10M
100M
INP
UT
RE
S
IS
T
A
NCE
(
)
FREQUENCY (Hz)
RS = 50, RFB = 200, CSH = 70pF
RS = 100, RFB = 400, CSH = 20pF
RS = 200, RFB = 800
RS = 500, RFB = 2k
07
03
0-
1
88
Figure 40. RIN vs. Frequency for Various Values of RFB
(Effects of RS and CSH Are Also Shown)
Note that at the lowest value (50 Ω), RIN peaks at frequencies
greater than 10 MHz. This is due to the BW roll-off of the LNA,
as mentioned previously.
However, as can be seen for larger RIN values, parasitic capacitance
starts rolling off the signal BW before the LNA can produce
peaking. CSH further degrades the match; therefore, CSH should
lists the recommended values for RFB and CSH in terms of RIN.
CFB is needed in series with RFB because the dc levels at Pin LO-x
and Pin LI-x are unequal.
Table 7. Active Termination External Component Values
LNA Gain
(dB)
RIN (Ω)
RFB (Ω)
Minimum
CSH (pF)
BW (MHz)
15.6
50
200
90
57
17.9
50
250
70
69
21.3
50
350
50
88
15.6
100
400
30
57
17.9
100
500
20
69
21.3
100
700
10
88
15.6
200
800
N/A
72
17.9
200
1000
N/A
72
21.3
200
1400
N/A
72
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