AD9271 Analog Devices, AD9271 Datasheet - Page 23

• Manufacturer Part Number: AD9271
• Description: Octal LNA/VGA/AAF/ADC and Crosspoint Switch
• Manufacturer: Analog Devices

Specifications of AD9271

• Resolution (bits): 12bit
• # Chan: 8
• Sample Rate: 50MSPS
• Interface: LVDS,Ser
• Analog Input Type: SE-Uni
• Ain Range: 0.25 V p-p,0.32 V p-p,0.4 V p-p
• Adc Architecture: Pipelined
• Pkg Type: QFP

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 R
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.
Figure 45 shows the noise figure as it relates to R
of R
Figure 44. Noise Figure vs. R
IN
Matched, and Unterminated Inputs, V
Active Termination Matched Inputs, V
, which is helpful for design purposes.
Figure 45. Noise Figure vs. R
16
14
12
10
16
14
12
10
8
6
4
2
0
8
6
4
2
0
10
10
S
for Resistive Termination, Active Termination
R
IN
UNTERMINATED
= 200Ω
S
for Various Fixed Values of R
R
R
R
100
S
100
S
R
R
IN
(Ω)
(Ω)
IN
IN
= 100Ω
Gain
Gain
= 50Ω
= 75Ω
UNTERMINATED
RESISTIVE TERMINATION
ACTIVE TERMINATION
= 1 V, 15.6 dB LNA Gain
= 1 V, 15.6 dB LNA Gain
S
to preserve the
S
for various values
IN
,
1000
1000
Rev. B | Page 23 of 60
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.
DRIVER
Tx
TRANSDUCER
Figure 46. Input Overload Protection
5kΩ
5kΩ
+5V
–5V
HV
2kΩ
BAS40-04
10nF
10nF
AD9271
AD9271
LNA