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TX810EVM: Rin resistor?

Jochen Rau
Prodigy 30 points

Part Number: TX810EVM

Hi!

I see in Figure 3 of the manual a Rin resistor of 75Ohm. 

Can anyone explain why this resistor there is required? If I attach a transducer there, the transmitter sees the transducer in parallel to the 75Ohm, so we'll loose some power into the resistor. Again on receive the transducer works with this resistor as the dominant load. 

I couldn't see any recommendation of such a resistor in the datasheet of the TX810, there the transducer only is (through a DC block capacitor) connected with the TX810. 

I would highly appreciate feedback here. Thanks!

  • 0
    TI__Genius 12025 points

    Hi,

    Thank you!

    we will reply to your question very soon.

    Best regards,

    Chen

  • 0 in reply to Chen Kung
    TI__Genius 12025 points

    Hi,

    Thanks for using TX810EVM.

    Those resistors are used for input signal impedance matching used.

    when your input sources are close to TX810 input

    then you can ignore these resistors.

    Thank you!

    Best regards,

    Chen

  • 0 in reply to Chen Kung
    Prodigy 30 points

    Hi! 

    Thanks for the swift answer! 

    With "close to the TX810 input" you mean a "source impedance similar to the input impedance of the TX810" or do you mean "signal source is close, so signal path in meter(or cm) is small". 

    What is the input impedance of the TX810? I don't see a spec in the datasheet. 

    Thanks!

    Jochen

     

  • 0 in reply to Jochen Rau
    TI__Genius 12025 points

    Hi,

    Thank you for your concern.

    For your concern about the input resistor load issue,

    our system suggests:

    Please refer to the TX7316.

    This may fit your need.

    Thank you!

    Best regards,

    Chen

  • 0 in reply to Chen Kung
    Prodigy 30 points

    Hi! 

    Sorry, but that does not match the question I posed. I asked a follow-up question to your answer: 

    Those resistors are used for input signal impedance matching used.

    when your input sources are close to TX810 input

    then you can ignore these resistors.

    So I wondered what you meant by "when your input sources are close to TX810 input". And posed the following questions: 

    With "close to the TX810 input" you mean a "source impedance similar to the input impedance of the TX810" or do you mean "signal source is close, so signal path in meter(or cm) is small". 

    What is the input impedance of the TX810? I don't see a spec in the datasheet. 

    Thanks.

    Jochen

  • 0 in reply to Jochen Rau
    TI__Genius 12025 points

    Hi,

    According to the TX810 datasheet,

    from the Figure 1 block diagram and Figure 3,

    and also compare to Table 1 (B2B2B1 = 000 to 111)

    you can see all the Ron impedance not exactly the same value.

    Therefore using 75ohm to match the internal impedance ~150ohm to ground (this is one option)

    so totally the input impedance is 50ohm but which is hard to be all the same conditions

    for all the conditions as B2B2B1 = 000 to 111.

    If you need very accurate input condition, then we recommend please select the new device as more option.

    That is why we recommend a newer option.

    Thank you!

    Best regards,

    Chen