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TDC1000-TDC7200EVM: Impedance match of low cost transducer

Thibault Helle
Expert 2090 points

Part Number: TDC1000-TDC7200EVM

Hi,

I am using the TDC1000-7200 EVM for flow mode and I have a low cost transducer with an impedance of 150 ohm which makes the excitation voltage go up to about 2,2 V instead of 3,6 V (like with a 200 ohm transducer). 

Do you have any suggestions to match the impedance (without removing the SMD resistor on the board) for maximal excitation power? 

Thanks in advance! 

Best regards,

Thibault

  • 0
    TI__Mastermind 31655 points
    Hi Thibault,

    Can you add a resistor? You can stack a 600ohm resistor on top of the 200ohm to achieve a parallel combination of 150ohm. Other than that you will have to start making modifications to the board to match the impedance.

    Regards,
  • 0 in reply to Scott Cummins
    Expert 2090 points

    Scott Cummins said:
    Hi Thibault,

    Can you add a resistor? You can stack a 600ohm resistor on top of the 200ohm to achieve a parallel combination of 150ohm. Other than that you will have to start making modifications to the board to match the impedance.

    Regards,

    Thanks for your answer, 

    I stacked a 600 ohm resistor as you instructed and it did increase the signal, but only by 100mV unfortunately. I then went over to think about the capacitance as it was much larger difference there. 

    Test 1: I tried to add a capacitor in series with the transducer line (the transducer has 1200pF) to match the 300pF on the board. I measured the Tx excitation signal and got a big nice increase of 1V and the signal looked good. However the echo was now much worse and barely visible...

    Test 2: So instead I tried to stack on a capacitor of 900pF in parallel with the on-board capacitor(C43). The echo signal increased by 50% but the excitation Tx signal did not change at all...

    I would really like to have the same Tx signal as in test 1 together with a better echo signal. I'm a bit confused about what is happening, could you explain this ? 
    Tell me if you need a better explanation of my experiments. (I am measuring the excitation signal on the socket J5 and echo on comp-in).

    Thank you

    Best regards,
    Thibault