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TDC1000 Cross-talk

Steven Lindholm
Prodigy 80 points
Other Parts Discussed in Thread: TDC1000, TDC7200

We are working with the TDC1000 and the TDC7200 to create a residential water metering device.  We thought we had everything working well, until we noticed that the timing was too consistent at different flows.  Going backwards, we started listening to the pulses through a steel block (108 mm long, but irrelevant to this question).  Set up is per the datasheet (minus the additional bandpass filter at this point) with XDC1 being TX1/RX2 and XDC2 being RX1/TX2.

What we noticed was an echo at about 4 us, which is about 14 us faster than the signal should arrive.  Disconnecting the transducers and placing an oscilliscope on the RX1 line, we saw the same echo at 4 us after the TX pulse was sent.  By my calculations, 4 us is just about the minimum space between the TX pulse and time-out if all of the short TOF parameters are left at default.

The TDC 1000 is configured for Mode 1, short TOF.

Is this a characteristic of the chip, a problem with the chip we have, or a problem with one of the register settings?

  • 0
    TI__Expert 3350 points

    I have observed three types of cross-talk in the flow application; 1) cross-talk in the pipe of the flow sensor, 2)  cross-talk  on the PCB, and finally some cross-talk internal to the IC. The internal cross-talk is attenuated by over 100 dB  and is insignificant (almost negligible). The  cross-talk on the board can be minimized with good layout practice. Depending on the design and the material that the ultrasonic sensor is made of, the cross-talk in the sensor can play a significant role. As I do not have the details of your system, it would be difficult to determine which of the last two sources of cross-talk is effecting your system most. I would suspect that if the sensor used in this application is metallic and has one end of each transducer grounded to the pipe (i.e. both ends of the transducers are not  floating), then the sensor is the reason for the cross-talk that you are experiencing.

  • 0 in reply to Bahram Mirshab
    Prodigy 80 points

    Right now the TDC1000 is mounted on a SOIC-to-DIP adapter board and the connections to the TDC 7200/microprocessor are through a quick proto breadboard (ref. OSEPP LS-00019).  The negative leads are brought back to the breadboard ground.  WE are not yet to the stage of creating a dedicated board; even then, all transducer grounds will be brought back to the board

    The implication I see here is that the setting for the Threshold detection may be too low (if we are picking up a signal attenuated by 100dB)  OR our gain is too high.  Certainly while sending through the steel we should need very little gain and very high threshold (we should have nearly 95% transmission through steel @ 90 degrees to the surface)

    Our oscilliscope measurements are made at the RX1 port; I presume then the cross-talk between TX1 and RX1 is before the LNA; so we get a 20 dB boost to the internal cross-talk before we get to the PGA section.

    Any suggestion on possible mask settings to prevent the cross-talk from being detected by the comparator?  Does this cross-talk within the chip extend in time with additional TX pulses? (thus increasing the required masking)

  • 0 in reply to Steven Lindholm
    TI__Expert 3350 points

    Please attach a scope shot of the echo for the upstream and the downstream signals as well as your GUI config file. a picture of your set up would also help. In the mean time please try "short time of flight"  and a higher threshold setting, say -410 mv to see if the cross talk signals can be avoided.