參數(shù)資料
型號(hào): AD9277-50EBZ
廠商: Analog Devices Inc
文件頁數(shù): 17/48頁
文件大小: 0K
描述: BOARD EVAL FOR AD9277
設(shè)計(jì)資源: AD9276/77 Eval Brd Gerber Files
標(biāo)準(zhǔn)包裝: 1
ADC 的數(shù)量: 1
位數(shù): 14
采樣率(每秒): 10M ~ 50M
數(shù)據(jù)接口: 串行,SPI?
輸入范圍: 733 mVpp
工作溫度: -40°C ~ 85°C
已用 IC / 零件: AD9277
已供物品:
相關(guān)產(chǎn)品: AD9277BSVZ-ND - IC ADC 14BIT LNA/VGA/AAF 100TQFP
AD9277
Rev. 0 | Page 24 of 48
LNA Noise
The short-circuit noise voltage (input-referred noise) is an
important limit on system performance. The short-circuit noise
voltage for the LNA is 0.75 nV/√Hz at a gain of 21.3 dB, including
the VGA noise at a VGA postamp gain of 27 dB. These measure-
ments, which were taken without a feedback resistor, provide
the basis for calculating the input noise and noise figure (NF)
performance of the configurations shown in Figure 49.
VOUT
UNTERMINATED
+
LI-x
RIN
RS
VOUT
RESISTIVE TERMINATION
+
LI-x
RIN
RS
VOUT
ACTIVE IMPEDANCE MATCH
+
LI-x
RIN
RFB
1 + A/2
RS
RIN =
08
18
1-
0
43
Figure 49. Input Configurations
Figure 50 and Figure 51 are simulations of noise figure vs. RS
results using these configurations and an input-referred noise
voltage of 3.8 nV/√Hz for the VGA. Unterminated (RFB = ∞)
operation exhibits the lowest equivalent input noise and noise
figure. Figure 51 shows the noise figure vs. source resistance
rising at low RS—where the LNA voltage noise is large compared
with the source noise—and at high RS due to the noise contribution
from RFB. The lowest NF is achieved when RS matches RIN.
The main purpose of input impedance matching is to improve the
transient response of the system. With resistive termination, the
input noise increases due to the thermal noise of the matching
resistor and the increased contribution of the LNA’s input
voltage noise generator. With active impedance matching,
however, the contributions of both are smaller (by a factor of
1/(1 + LNA gain)) than they would be for resistive termination.
Figure 50 shows the relative noise figure performance. With an
LNA gain of 21.3 dB, the input impedance was swept with RS to
preserve the match at each point. The noise figures for a source
impedance of 50 Ω are 7.3 dB, 4.2 dB, and 2.8 dB for the resistive
termination, active termination, and unterminated configurations,
respectively. The noise figures for 200 Ω are 4.5 dB, 1.7 dB, and
1.0 dB, respectively.
Figure 51 shows the noise figure as it relates to RS for various
values of RIN, which is helpful for design purposes.
10
100
1k
0
1.5
3.0
4.5
6.0
7.5
9.0
10.5
12.0
RS ()
NO
IS
E
F
IG
U
RE
(
d
B)
UNTERMINATED
RESISTIVE TERMINATION
ACTIVE TERMINATION
08
18
1-
0
44
Figure 50. Noise Figure vs. RS for Resistive Termination, Active
Termination Matched, and Unterminated Inputs, VGAIN = 0.8 V
10
100
1k
0
1
2
3
4
5
6
7
8
RS ()
NO
IS
E
F
IG
U
RE
(
d
B)
RIN = 50
RIN = 75
RIN = 100
RIN = 200
UNTERMINATED
08
18
1-
0
45
Figure 51. Noise Figure vs. RS for Various Fixed Values of RIN,
Active Termination Matched Inputs, VGAIN = 0.8 V
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