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AFE5809 Datasheet(PDF) 78 Page - Texas Instruments |
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AFE5809 Datasheet(HTML) 78 Page - Texas Instruments |
78 / 100 page (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 = + ´ AFE5809 SLOS738D – SEPTEMBER 2012 – REVISED JANUARY 2014 www.ti.com CM_BYP and VHIGH pins, which generate internal reference voltages, need to be decoupled with ≥ 1-µF capacitors. Bigger bypassing capacitors (> 2.2 µF) 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 AFE5809 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. (12) The AFE5809 achieves low noise figure (NF) over a wide range of source resistances as shown in Figure 32, Figure 33, and Figure 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 99 shows three termination configurations: Figure 99. Termination Configurations 78 Submit Documentation Feedback Copyright © 2012–2014, Texas Instruments Incorporated Product Folder Links: AFE5809 |
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