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TDC1000: Matching impedance of ultrasonic sensors for the electroacoustic reciprocity in flow meter

Part Number: TDC1000


We are designing the ultrasonic flow meter on the TI chipset and struggling with Zero-flow-offsets issues.

Section 6.1.2 Improving Zero-flow-offset and Offset Drift Over Temperature of the  app note seems to be helpful in this regard.  

It says: "The electrical impedance of the path can be controlled using an impedance matching solution, based on the principal of electroacoustic reciprocity. A well implemented impedance matching feature results in the operation of the circuit in a "sufficiently reciprocal" way".

Well, cool, electroacoustic reciprocity it's what we need for sure need....

But then I see the schematics being offered on page 9, and all it is is basically a 300 pf capacitor in transmit mode parallel to the piezo (see the pic below). And also a 200 Ohm resistor is shunting the receiving piezo, which  seems to be just damping the "ringing" (and the useful signal as well).  

Looking at the equivalent schematics of the piezo and it's resonance and anti-resonance formulas it looks like it's resonance is defined just by  L1 and C1, and not by parallel capacitor (which affects anti-resonance only). So adding extra 300pf at transmit mode will likely not change resonant frequency and piezo's behavior while transmitting.  

In conclusion, I am wondering how the proposed solution can help with the electroacoustic reciprocity and how exactly this proposed matching works

Thank you for your time,  


  • Hi Dmitry,

    The TDC1000 is no longer recommended for flow metering applications. Recommended alternative implementations use the MSP430FR6047 for ultrasonic flow monitoring, see this link for reference design: Design Guide

    The TDC1000 has issues in accurately reporting low flow rates, and has potential issues where the TDC1000 does not create stop pulses. 

    See this E2E post for further information on the MSP solution compared to discrete approaches: E2E Post

    I am not exactly certain how the impedance matching is being achieved in the application note you included. I suspect the impedance matching in the app note is from the identical  impedance/trace length in the Rx and Tx path due to the switching circuit. Otherwise, it looks like the passive components will be damping the burst as you already commented. Some of the performance gains realized here may be attributed to unique characteristics of the TDC1000.



  • Hi Jacob

    I am not using TDC1000, I mentioned it just because of the app note that contained the electroacoustic reciprocity section with questionable impedance matching method.



  • Thank you for the clarification.

    Feel free to make another E2E post if you have any further questions.