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I'm new to signal processing and have a specific question about a PCB design by TI. (TDC7200EVM User’s Guide ) (TDC7200EVM)

Could anyone explain why they place the series terminating resistors on the receiver side?

Is it not better to use parallel termination in this case?

Or am I completely wrong and these aren't terminating resistors?

TDC7200EVM PCB design

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  • \$\begingroup\$ The TDC7200 datasheet says at the layout considerations: The length of the START and STOP traces from the TDC7200 to the stopwatch/MCU should be matched to prevent uneven signal delays. Also, avoid unnecessary via-holes on these traces and keep the routing as short/direct as possible to minimize parasitic capacitance on the PCB. I think they added R1 and R2 to get a better match. These 'big' resistors nihilate differences that are an order smaller, like solder joint resistance etc. \$\endgroup\$
    – Huisman
    Commented Dec 28, 2019 at 13:06
  • \$\begingroup\$ I can't figure out a good reason for those resistors, unless it's somehow related to the fact that the STOP & START inputs are Schmitt trigger inputs. Also, their value of 49.9 ohms implies that they're 0.1% precision, which is way overkill for any kind of impedance matching use. \$\endgroup\$
    – SteveSh
    Commented Dec 28, 2019 at 13:57

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Thanks a lot for your answers.
Texas Instruments kindly explained the reason for those resistors:

The in-line resistance acts as part of a low-pass filter given the input capacitance is 3pF at the START and STOP pins. This is to prevent false-positives otherwise created by high-frequency transients-glitches at these pins.

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  • \$\begingroup\$ Thanks for sharing the answer from TI. \$\endgroup\$
    – SteveSh
    Commented Jan 2, 2020 at 13:29

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