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參數(shù)資料
| 型號： | AD9271BSVZRL-50 |
| 廠商： | Analog Devices Inc |
| 文件頁數(shù)： | 16/60頁 |
| 文件大?。?/td> | 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 個單端，單極；8 個差分，單極 |
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AD9271
Rev. B | Page 23 of
60
0
06
30
4-
10
3
NO
IS
E
F
IG
URE
(
d
B
)
2
4
6
8
10
12
14
16
10
100
1000
RS ()
UNTERMINATED
RESISTIVE TERMINATION
ACTIVE TERMINATION
Figure 44. Noise Figure vs. RS for Resistive Termination, Active Termination
Matched, and Unterminated Inputs, VGain = 1 V, 15.6 dB LNA Gain
06
30
4-
1
0
10
100
1000
RS ()
02
2
4
6
8
10
12
14
16
NO
IS
E
F
IG
UR
E
(
d
B)
RIN = 50
RIN = 100
RIN = 75
R
= 200
IN
UNTERMINATED
Figure 45. Noise Figure vs. RS for Various Fixed Values of RIN,
Active Termination Matched Inputs, VGain = 1 V, 15.6 dB LNA Gain
The primary 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 44 shows the relative noise figure performance. In this
graph, 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 7.1 dB, 4.1 dB, and 2.5 dB for the resistive termination,
active termination, and unterminated configurations, respectively.
The noise figures for 200 Ω are 4.6 dB, 2.0 dB, and 1.0 dB,
respectively.
of RIN, which is helpful for design purposes.
INPUT OVERDRIVE
Excellent overload behavior is of primary importance in ultra-
sound. Both the LNA and VGA have built-in overdrive
protection and quickly recover after an overload event.
Input Overload Protection
As with any amplifier, voltage clamping prior to the inputs is
highly recommended if the application is subject to high
transient voltages.
A block diagram of a simplified ultrasound transducer interface
is shown in Figure 46. A common transducer element serves the
dual functions of transmitting and receiving ultrasound energy.
During the transmitting phase, high voltage pulses are applied
to the ceramic elements. A typical transmit/receive (T/R) switch
can consist of four high voltage diodes in a bridge configuration.
Although the diodes ideally block transmit pulses from the
sensitive receiver input, diode characteristics are not ideal, and
resulting leakage transients imposed on the LI-x inputs can be
problematic.
Because ultrasound is a pulse system and time-of-flight is used
to determine depth, quick recovery from input overloads is
essential. Overload can occur in the preamp and the VGA.
Immediately following a transmit pulse, the typical VGA gains
are low, and the LNA is subject to overload from T/R switch
leakage. With increasing gain, the VGA can become overloaded
due to strong echoes that occur near field echoes and
acoustically dense materials, such as bone.
Figure 46 illustrates an external overload protection scheme. A
pair of back-to-back Schottky diodes is installed prior to installing
the ac-coupling capacitors. Although the BAS40 diodes are shown,
any diode is prone to exhibiting some amount of shot noise. Many
types of diodes are available for achieving the desired noise per-
formance. The configuration shown in Figure 46 tends to add
2 nV/√Hz of input-referred noise. Decreasing the 5 kΩ resistor
and increasing the 2 kΩ resistor may improve noise contribution,
depending on the application. With the diodes shown in Figure 46,
clamping levels of ±0.5 V or less significantly enhance the
overload performance of the system.
0
63
04
-10
0
TRANSDUCER
10nF
2k
5k
5k
AD9271
Tx
DRIVER
HV
BAS40-04
+5V
–5V
LNA
Figure 46. Input Overload Protection
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