Datasheet

AD9279
Rev. 0 | Page 25 of 44
The main purpose of input impedance matching is to improve the
transient response of the system. With shunt termination, the input
noise increases due to the thermal noise of the matching resistor
and the increased contribution of the LNA input voltage noise
generator. With active termination, however, the contributions
of both are smaller (by a factor of 1/(1 + LNA Gain)) than they
would be for shunt termination.
Figure 48 shows the relative noise figure performance. With an
LNA gain of 21.3 dB, the input impedance was swept with R
S
to
preserve the match at each point. The noise figures for a source
impedance of 50  are 7.3 dB, 4.2 dB, and 2.8 dB for the shunt
termination, active termination, and unterminated configurations,
respectively. The noise figures for 200  are 4.5 dB, 1.7 dB, and
1.0 dB, respectively.
Figure 49 shows the noise figure as it relates to R
S
for various
values of R
IN
, which is helpful for design purposes.
10 100 1k
0
1.5
3.0
4.5
6.0
7.5
9.0
10.5
12.0
R
S
(Ω)
NOISE FIGURE (dB)
UNTERMINATED
ACTIVE TERMINATION
09423-042
SHUNT TERMINATION
Figure 48. Noise Figure vs. R
S
for Shunt Termination, Active
Termination Matched and Unterminated Inputs, V
GAIN
= 1.6 V
10 100 1k
0
1
2
3
4
5
6
7
8
R
S
(Ω)
NOISE FIGURE (dB)
R
IN
= 50Ω
R
IN
= 75Ω
R
IN
= 100Ω
R
IN
= 200Ω
UNTERMINATED
09423-043
Figure 49. Noise Figure vs. R
S
for Various Fixed Values of R
IN
,
Active Termination Matched Inputs, V
GAIN
= 1.6 V
Input Overdrive
Excellent overload behavior is of primary importance in
ultrasound. Both the LNA and VGA have built-in overdrive
protection and quickly recover after an overload event.
As with any amplifier, voltage clamping prior to the inputs
is highly recommended if the application is subject to high
transient voltages.
Figure 50 shows a simplified ultrasound transducer interface.
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 the resulting leakage
transients imposed on the LI-x inputs can be problematic.
The external input overload protection scheme also contains a
pair of back-to-back signal diodes that should be in place prior to
the ac coupling capacitors. Keep in mind that all diodes are prone
to exhibiting some amount of shot noise. Many types of diodes are
available for achieving the desired noise performance. The
configuration shown in Figure 50 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 50, clamping
levels of ±0.5 V or less significantly enhance the overload
performance of the system.
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 preamplifier and in 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.
TRANSDUCER
10nF
10nF
2kΩ
5kΩ
5kΩ
AD9279
Tx
DRIVER
HV
+5
V
–5V
LNA
09423-044
Figure 50. Input Overload Protection