AFE5808A datasheet(49/66 Pages) TI1

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LNAnoise

LNA _ Noise

(b) Active Termination

S0499-01

LNA

(a)No Termination

(c)Passive Termination

LNA

AFE5808A

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SLOS729B – OCTOBER 2011 – REVISED APRIL 2012

CM_BYP and VHIGH pins, which generate internal reference voltages, need to be decoupled with

≥1uF

capacitors. Bigger bypassing capacitors (>2.2uF) 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

AFE5808A achieves low power and an exceptionally low-noise voltage of 0.63 nV/

√Hz, and a low current noise

of 2.7 pA/

√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 AFE5808A 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 84 shows three termination configurations:

Figure 84. Termination Configurations

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AFE5808A Datasheet(PDF) 49 Page - Texas Instruments