Datasheet

(b) Active Termination
S0499-01
LNA
R
s
(a)No Termination
(c)Passive Termination
LNA
R
s
R
f
LNA
R
s
R
t
2 2 2
total LNAnoise s LNAnoise
LNA _ Noise V R I= + ´
AFE5808
www.ti.com SLOS688C –SEPTEMBER 2010–REVISED APRIL 2012
CM_BYP and VHIGH pins, which generate internal reference voltages, need to be decoupled with ≥1µF
capacitors. Bigger bypassing capacitors (>2.2µF) may be beneficial if low frequency noise exists in system.
LNA Noise Contribution
The noise spec is critical for LNA and it determines the dynamic range of entire system. The LNA of the
AFE5808 achieves low power and an exceptionally low-noise voltage of 0.63nV/√Hz, and a low current noise of
2.7pA/√Hz.
Typical ultrasonic transducer’s impedance Rs varies from tens of ohms to several hundreds of ohms. Voltage
noise is the dominant noise in most cases; however, the LNA current noise flowing through the source
impedance (Rs) generates additional voltage noise.
(5)
The AFE5808 achieves low noise figure (NF) over a wide range of source resistances as shown in Figure 32,
Figure 33, andFigure 34.
Active Termination
In ultrasound applications, signal reflection exists due to long cables between transducer and system. The
reflection results in extra ringing added to echo signals in PW mode. Since the axial resolution depends on echo
signal length, such ringing effect can degrade the axial resolution. Hence, either passive termination or active
termination, is preferred if good axial resolution is desired. Figure 83 shows three termination configurations:
Figure 83. Termination Configurations
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