shows an example of how to select RFB1 and R
參數(shù)資料
型號(hào)： AD9279-80KITZ
廠商： Analog Devices Inc
文件頁(yè)數(shù)： 17/44頁(yè)
文件大?。?/td> 0K
描述： KIT EVALUATION FOR AD9279
標(biāo)準(zhǔn)包裝： 1
ADC 的數(shù)量： 8
位數(shù)： 12
采樣率（每秒）： 80M
數(shù)據(jù)接口： 串行
工作溫度： -40°C ~ 85°C
已用 IC / 零件： AD9279
已供物品： 板
AD9279
Rev. 0 | Page 24 of 44
shows an example of how to select RFB1 and RFB2 for 66 Ω, 100 Ω,
and 200 Ω input impedance for LNA gain = 21.3 dB (12×).
Table 9. Active Termination Example for LNA Gain = 21.3 dB,
RFB1 = 700 Ω, RFB2 = 1400 Ω
Register
0x2C
Value
Rs (Ω)
LO-x
Switch
LOSW-x
Switch
RFB (Ω)
RIN (Ω)
(Eq. 1)
00
(default)
100
On
Off
RFB1
100
01
50
On
RFB1||RFB2
66
10
200
Off
On
RFB2
200
11
N/A
Off
∞
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.
Figure 46 shows RIN vs. frequency for various values of RFB.
09
42
3-
0
40
10
100
1k
100k
1M
10M
100M
IN
PU
T
R
ESI
S
T
A
N
C
E
(
)
FREQUENCY (Hz)
RS = 50, RFB = 200, CSH = 70pF
RS = 100, RFB = 400, CSH = 20pF
RS = 200, RFB = 800
RS = 500, RFB = 2k
Figure 46. RIN vs. Frequency for Various Values of RFB
(Effects of RSH and CSH Are Also Shown)
Note that, at the lowest value of RIN (50 Ω), RIN peaks at frequencies
greater than 10 MHz. This is due to the BW roll-off of the LNA.
However, as can be seen for larger RIN values, parasitic capaci-
tance starts rolling off the signal BW before the LNA can produce
peaking. CSH further degrades the match; therefore, CSH should
not be used for values of RIN that are greater than 100 Ω.
Table 10 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 10. 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
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 47.
VOUT
UNTERMINATED
+
–
LI-x
RIN
RS
VOUT
SHUNT TERMINATION
+
–
LI-x
RIN
RS
VOUT
ACTIVE TERMINATION
+
–
LI-x
RIN
RFB
1 + A/2
RS
RIN =
09
42
3-
0
41
Figure 47. Input Configurations
Figure 48 and Figure 49 are simulations of noise figure vs. RS
results using these configurations and an input-referred noise
voltage of 2.5 nV/√Hz for the VGA. Unterminated (RFB = ∞)
operation exhibits the lowest equivalent input noise and noise
figure. Figure 49 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.
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