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TDC1000: 180° shifted long ringing of the transducer, even if the 180° pulse shift option is disabled

Mikel Gorostiaga
Prodigy 50 points
Part Number: TDC1000

Hello Everybody,

I am using the TDC1000 ASIC for liqued level measuring applications. So far the system works, but I am trying to increase the resolution by improving the excitation signal of the transducer. Regarding this topic, I have two similar but separate questions.

1-I looked at the ASIC schematics available at the TI website. The evaluation kit schematics of the TDC1000 show that there are a 200ohm resistor in series (R43 or R45) and a 510ohm resistor in parallel (R40 or R57) between the emitting transducer and the ASIC. By contrast, the TIDA00322 schematics for the non-boosted transducer (TX2) show that only a 110ohm resistor in series (R39) is used between emitting transducer an ASIC. In Receiving modus a 300pF in series is used in both schematics.

Why are the schematics for the emitting transducer different? Is the 510ohm parallel resistor a bleeder resistor to decrease ringing?

The second issue arises from the question above:

2-Initially, I had a 220 resistor in series as in the schematics, but the excitation voltage at the 2MHz transducer decreases to about half of its value due to the resistor. Therefore, I decided to connect the transducer directly to the ASIC. In addition, I also made an L-network with Series Inductor electric matching circuit (non-ideal)  to decrease the transducer ringing due to electric impedance mismatch between ASIC (75ohm) and the transducer.(see impedances in TI_impedances.tif).

TI_Impedance.tif

I have measured the voltage at the transducer with 5 Pulses excitation and with the transducer directly connected to the ASIC and with the matching circuit (see TI_Emission.tif). The unmatched transducer has a bigger ringing than the unmatched transducer, which I was expecting. However, the ringing is 180° shifted compared to the excitation signal, even though the 180° pulse shift and  the damping  options of the ASIC is disabled. Finally, I think that the 200kHz component of the signals is due to the 1st radial mode of the transducer.

TI_Emission.tif

Why is the ringing of the unmatched transducer 180° shifted compared to the excitation signal? Does the ASIC automatically try to attenuate the ringing? Any help in these issues would be greatly appreciated.

Kind regards,

Mikel

  • 0
    TI__Genius 11015 points
    Mikel,

    I have received your question & will respond in a couple of days.
  • 0 in reply to Bharat Aravamudhan
    Prodigy 50 points

    Hello Bharat,

    I have been doing some experiments and research on the topic and I might have found the reason for the 180° phase shift and the parallel resistor at the transducer:

    • When emitting, the ASIC pin has a low impedance value, which is also close to the value of that of my transducer at Resonance (20ohm approx.). Thus the Reflection value is close to 0. However, when the signal finishes, the emitting pin of the ASIC changes to high impedance, which creates a Reflection of -1 and thus the 180° shift occurs. This might be the explanation for that. What do you think?

    Coincidentally, this 180° shift creates a destructive interference on the acoustic part, and the measured reflection is shorter than the ringing length.

    • The resistor in parallel to the transducer (in my case I have added a 500ohm resistor) helps to decrease the DC and low frequency ringing values. Still, I do not know why the schematics are different for TIDA and the Eval Boards. I suppose that each transducer requires its own optimized schematics.

    Regarding all these questions, I have checked the DataSheet and I have seen an "upper limit value" neither for the current for the ASIC in output mode  Iout(tx) nor for Vout(tx). Are there any limits in Transmitter Output for the ASIC? This might be critical, if the Transducer is directly connected to the ASIC and high voltages are achieved due to standing waves.

    Thanks in advance for your time and effort.

    Regards,

    Mikel

  • 0 in reply to Mikel Gorostiaga
    TI__Genius 11015 points
    Mike,

    I am working on this request and I will respond to you within the next couple of days. thanks for your patience.
  • 0 in reply to Mikel Gorostiaga
    TI__Genius 11015 points
    Mikel,

    Thanks for your patience. You explanation on the 180 deg phase shift is correct.
    The resistor values in each of the EVM/Reference design is setup based on the transducer used and application it is intended to and hence the cap & resistance values differ.
    The datasheet does not provide any current limiting value at any output pins, it just specifies the absolute max input current at any pin to be a max of 5mA. The device cannot out put more than ~3.3V through the Rx pin.
  • 0 in reply to Bharat Aravamudhan
    Prodigy 50 points

    Thanks Bharat for your response.

    So to be absolutely clear here, the negative voltages shown in TI_Signal.tif (emission signal) will not damage the ASIC? Or would you recommend adding a protective diode at the Tx pin?  Same question for the RX pin that was in open-state while this voltage was measured accross the transducer electrodes.

    Concerning the 5mA max input current, the voltage of the measured echo is much lower and I will not such current input at Rx in any circunstances.

    6470.TI_SignalVoltage.tif

    Thanks for your help and patience.

    Regards,

    Mikel

  • 0 in reply to Mikel Gorostiaga
    TI__Genius 11015 points
    Mikel,

    Please send me a schematic you your design to be extra clear on your setup.

    In case you follow a setup say similar to the TDC1000-TDC7200 EVM you should not need any diode at the Tx except a small resistance in series and a capacitance in the Rx path to filter the DC component should be sufficient.
  • 0 in reply to Bharat Aravamudhan
    Prodigy 50 points

    Hi Bharat,

    Thanks for your response.

    My circuit is very simple (see pic): I have directly connected my transducer to the TX pin of the ASIC. There is also a resistor of 560ohm connected in parallel between the transducer and ground, to decrease the ringing. I have experimentally determined the value of this resistor by looking at the trade-off between ringing at emission and decrease in echo-measuring sensitivity.

    As for the RX pin, I have added a 300pF capacitor in series as explained by the data-sheet.

    I do not have a series resistor at the TX pin, because it would greatly decrease the performance of the system. This is obvious since I would divide the available emitting power between this series resistor and the transducer. Also bear in mind that my transducer has 22ohm in resonsnace; 38ohm when loaded with the front acoustic material.

    Originally, that is my question: What is that series resistor for? Protection?

    If so, I would instead add a matching circuit, which would not influence the emitting voltage so much (but at the cost of reducing the sensitivity). (Picture of emitting voltage attached)

    Sktech.tifbleeder vs matched.tifwithSeriesResistor.tif

    Thanks for your support

    Cheers

    Mikel

  • 0 in reply to Mikel Gorostiaga
    TI__Genius 11015 points
    Mikel,

    The series resistor at the Tx is only a current limiting resistor and can be avoided.
  • 0 in reply to Bharat Aravamudhan
    Prodigy 50 points

    Thank you very much Bharat for your support!

    Mikel