參數(shù)資料
型號(hào)： AD9271BSVZRL-50
廠商： Analog Devices Inc
文件頁(yè)數(shù)： 15/60頁(yè)
文件大小： 0K
描述： IC ADC OCT 12BIT 50MSPS 100-TQFP
標(biāo)準(zhǔn)包裝： 1,000
位數(shù)： 12
采樣率（每秒）： 50M
數(shù)據(jù)接口： 串行，SPI?
轉(zhuǎn)換器數(shù)目： 8
功率耗散（最大）： 1.49W
電壓電源： 單電源
工作溫度： -40°C ~ 85°C
安裝類型： 表面貼裝
封裝/外殼： 100-TQFP 裸露焊盤
供應(yīng)商設(shè)備封裝： 100-TQFP-EP（14x14）
包裝： 帶卷 (TR)
輸入數(shù)目和類型： 8 個(gè)單端，單極；8 個(gè)差分，單極
AD9271
Rev. B | Page 22 of
60
Because the amplifier has a gain of 6× 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 9.6 dB (3×). The input resistance is
reduced by an internal bias resistor of 15 kΩ in parallel with the
source resistance connected to Pin LI-x, with Pin LG-x 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 is 845 Ω. If the
simplified equation (Equation 2) is used to calculate RIN, the
value is 190 Ω, resulting in a gain error less than 0.5 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 about 70 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 42 shows RIN vs. frequency for various values of RFB.
06
30
4-
10
10
100k
1M
10M
50M
FREQUENCY (Hz)
5
100
1k
INP
UT
I
M
P
E
DA
NCE
(
)
RS = 50, RFB = 200, CSH = 70pF
RS = 100, RFB = 400, CSH = 20pF
RS = 200, RFB = 800
RS = 500, RFB = 2k
Figure 42. RIN vs. Frequency for Various Values of RFB
(Effects of RSH and CSH Are Also Shown)
Note that at the lowest value, 50 Ω, in Figure 42, 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
not be used for values of RIN that are greater than 100 Ω. Table 7
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
RIN (Ω)
RFB (Ω)
Minimum
CSH (pF)
BW (MHz)
5×
50
175
90
49
6×
50
200
70
59
8×
50
250
50
73
5×
100
350
30
49
6×
100
400
20
59
8×
100
500
10
73
5×
200
700
N/A
49
6×
200
800
N/A
49
8×
200
1000
N/A
49
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 1.2 nV/√Hz or 1.4 nV/√Hz (at 15.6 dB
LNA gain), including the VGA noise. These measurements,
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 43. Figure 44 and Figure 45
are simulations of noise figure vs. RS results using these config-
urations and an input-referred noise voltage of 4 nV/√Hz for
the VGA. Unterminated (RFB = ∞) operation exhibits the lowest
equivalent input noise and noise figure. Figure 45 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.
VOUT
UNTERMINATED
+
–
VIN
RIN
RS
VOUT
RESISTIVE TERMINATION
+
–
VIN
RIN
RS
VOUT
ACTIVE IMPEDANCE MATCH
+
–
VIN
RIN
RFB
1 + A/2
RS
RIN =
06
30
4-
1
04
Figure 43. Input Configurations
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