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參數(shù)資料
| 型號： | AD9277-50EBZ |
| 廠商： | Analog Devices Inc |
| 文件頁數(shù)： | 16/48頁 |
| 文件大?。?/td> | 0K |
| 描述： | BOARD EVAL FOR AD9277 |
| 設(shè)計資源： | 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： | 板 |
| 相關(guān)產(chǎn)品： | AD9277BSVZ-ND - IC ADC 14BIT LNA/VGA/AAF 100TQFP |
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AD9277
Rev. 0 | Page 23 of 48
Low value feedback resistors and the current-driving capability
of the output stage allow the LNA to achieve a low input-
referred noise voltage of 0.75 nV/√Hz (at a gain of 21.3 dB).
This is achieved with a current consumption of only 27 mA per
channel (80 mW). On-chip resistor matching results in precise
single-ended gains, which are critical for accurate impedance
control. The use of a fully differential topology and negative
feedback minimizes distortion. Low second-order harmonic
distortion is particularly important in second harmonic ultra-
sound imaging applications. Differential signaling enables
smaller swings at each output, further reducing third-order
harmonic distortion.
Active Impedance Matching
The LNA consists of a single-ended voltage gain amplifier with
differential outputs and the negative output externally available.
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.
RFB is the resulting impedance of the RFB1 and RFB2 combination
(see Figure 47).
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 that it is 6 dB less
than the gain of the amplifier, or 11.9 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, with
Pin LG-x ac grounded. Equation 2 can be used to calculate the
required 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 of less than 0.6 dB.
Some factors, such as the presence of a dynamic source resistance,
may influence the absolute gain accuracy more significantly. At
higher frequencies, the input capacitance of the LNA must 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.
08
18
1-
0
42
10
100
1k
100k
1M
10M
100M
INP
U
T
RE
S
IS
T
ANCE
(
)
FREQUENCY (Hz)
RS = 50, RFB = 200, CSH = 70pF
RS = 100, RFB = 400, CSH = 20pF
RS = 200, RFB = 800
RS = 500, RFB = 2k
Figure 48. RIN vs. Frequency for Various Values of RFB
(Effects of RS 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,
as mentioned previously.
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
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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