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TDC7200 multiple hits

Raiko Klemp
Prodigy 60 points
Other Parts Discussed in Thread: TDC7200

Hello,


we are using the TDC7200 for ultrasound measurements. To measure frequency changes we started to use multiple hits. In our case it pointed out, that the first hit is quite sensitive but the following hits have only a resolution of 1/MCLK = 62.5ns. So only digital jumps can be seen on later hits.

  • 0
    TI__Mastermind 21685 points
    Raiko,
    Your complete question didn't come through. Please check.
    Thanks,
    Vishy
  • 0 in reply to Vishy Viswanathan
    Prodigy 60 points

    Good morning,

    We want to use 5 hits. If we change the signal by some nano seconds the changes can only be seen on the first hit. The other hits just show the random noise which is also present on the first hit. But if the changes in the signal are bigger than 62.5ns a jump on the other hits can be observed. This does not happen at the same time because the signal frequency is not a multiple of the main clock. That means it can be that some stops happen in the middle of a rising edge of the main clock, some at the beginning or end. If the time changes is between a falling vs. rising edge of the main clock only changes on the first hit can be observed.


    We also checked if the reason is the first trigger so we set the tdc delay to a value that we are somewhere in the middle of our triggers. The same was observed.

    Best.

  • 0 in reply to Raiko Klemp
    TI__Mastermind 21685 points
    Raiko,
    Minimum time between two stop signals is 67ns (mode 1) for TDC7200. May be that's the constraint you are seeing? See Data Sheet page 5.
    Thanks,
    Vishy
  • 0 in reply to Vishy Viswanathan
    Prodigy 60 points
    Vishy,
    this is not the point. We are using mode 2. The times between the stop signals in the example (see plots) are about 700-800ns. Our point is the sensitivity of the hits. You can test it by connecting an AWG with trigger output to the chip. Connect trigger output to start and output with stop. Generate a rectangluar burst with e.g. 10 periods. Change the frequency. In general the expected time diffs between the hits should corresponde to the frequency periode. Than you should change the start phase by some degree. In our example such small changes can be seen on the first hit but on the others only if the phase jumps are big enough (seeable as hard jumps not smooth changes).