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TDC1000-TDC7200EVM: TOF Limitations

Tapiwa Mutasa
Prodigy 10 points

Part Number: TDC1000-TDC7200EVM

Hello all,

I am wondering what the minimum amount of time between pulse and echo that the TDC1000-TDC7200EVM is able to process would be.

I searched for a datasheet that gave me experimental values such as that, but I could not seem to find one.

I understand that usually the issue will be the reverberations of the pulse overlapping with the return echo, but I am wondering if there is a standard considered "limit" or just a total limit to the chip.

Thanks,
Cameron

  • 0
    Prodigy 70 points

    (I am the original poster, I was signed into the wrong account)

    Side note: I am aware that the TDC7200 can measure down to 12 nanoseconds (or even lower), however I do not know if that translates to the whole TDC1000-TDC7200 system, or if the 1000 raises that minimum to a higher value.

  • 0
    Prodigy 70 points

    Hello all,

    I am wondering what the minimum amount of time between pulse and echo that the TDC1000-TDC7200EVM is able to process would be.

    I searched for a datasheet that gave me experimental values such as that, but I could not seem to find one.

    I understand that usually the issue will be the reverberations of the pulse overlapping with the return echo, but I am wondering if there is a standard considered "limit" or just a total limit to the chip.

    Thanks,
    Cameron

  • 0 in reply to Cameron Keller
    TI__Expert 4720 points

    Hi Cameron,

    My apologies for the delay. Our resident TDC expert is traveling at the moment, but I can recommend an app note to help in the meantime: http://www.ti.com/lit/an/snaa270/snaa270.pdf

    Here are two things to note:

    • There’s a transducer ring down that can generate false echo and limits the minimum time that can be measured with TDC1000
    • By itself TDC1000 has a “short TOF” mode in which the receive section can be kept enabled to always look for echo.

    TDC1000 operation in “short TOF measurement” mode (datasheet Section 8.4.6) and transducer ring down effects are discussed in the above app note.

    Best regards,

    Blair

  • 0 in reply to Blair Conner
    Prodigy 70 points

    Hi Blair,

    Thanks for the response.

    I have checked out all the datasheets and am aware of the transducer ring down/false echo issue, as well as the "Force Short TOF" setting. I am more wondering if in a perfect situation, with no ring down, what the minimum TOF measurable would be. Such as, if there is a delay after the transmitted signal before it will record a "Stop". Or maybe if there isn't, how much of the START pulse must be recorded before it will record an overlapping STOP signal. 

    Thanks again,

    Cameron

  • 0 in reply to Cameron Keller
    TI__Mastermind 31655 points
    Hello Cameron,

    The TDC7200 is capable is of measuring down to 12ns minimum between the start and stop signals. Ultimately the minimum time measured when using the TDC1000 and TDC7200 together is going to come down to the transmit period of the TDC1000. Given a CLKIN frequency of 8Mhz, the maximum transmit frequency would be 4Mhz, equating to a period of 250ns. With a single TX pulse at 50% duty cycle, just the transmit period will take at least 125ns even with no ringing.

    Ultimately, the real system minimum will depend the most on the transmit settings chosen, and the transducer that is used.
  • 0 in reply to Scott Cummins
    Prodigy 70 points
    Thanks for the response Scott,

    So the masking period, or the "Short TOF Blank Period" somehow only masks the transmitted signal, and not the echo? Or is that able to be disabled, leading to the 250ns?

    Best regards,
    Cameron
  • 0 in reply to Cameron Keller
    TI__Mastermind 31655 points
    Hello Cameron,

    No, the blanking will not distinguish between transmitter ringdown and the receiving echo. I was just responding to the hypothetical perfect situation you mentioned with no ringdown. The theoretical perfect limit would be 250ns, but this is unobtainable in a real system because the ringdown will always exist.

    What is the minimum distance you want to measure, and in what medium?

    Regards,
  • 0 in reply to Scott Cummins
    Prodigy 70 points
    Scott,

    But in this hypothetical perfect situation, I will still be limited to at least a microsecond after the START signal due to the Blank Period, right? Or can it be disabled?

    As thin as I can, and in steel. I was able to get it down to around 2us, but that was using an oscilloscope, and adjusting the settings until the STOP signal landed where I knew it should.

    Thanks,
    Cameron
  • 0 in reply to Cameron Keller
    TI__Mastermind 31655 points
    Ah, yes. You are correct that you cannot disable the short TOF blanking entirely. The minimum can be lower than 1us if you use a higher frequency system clock. With the maximum CLKIN of 16Mhz, the blanking time can be 0.5us. This will be added to the minimum transmit time of now 125ns (I was a bit mistaken before as the duty cycle does not matter. The start pulse width will be equal to the T1 period for TX pulse numbers < 3). In this case the theoretical limit would be 750ns.

    Would it be possible to attach a shim to the transducer? It would be made of the same steel you are measuring, and of a known width. That way you can increase the TOF into a more reasonable range. Just subtract the time taken to travel through the shim to find the thickness of the actual measured piece.

    Regards,
  • 0 in reply to Scott Cummins
    Prodigy 70 points

    Not a bad idea at all. Thanks for the answer, Scott!

    Cameron

  • 0 in reply to Scott Cummins
    Prodigy 70 points
    I am unable to mark this post as answered due to being logged in to the wrong account originally, so feel free to do so yourself!