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TDC1000: TDC1000 for Zero Flow issue

Mickey Zhang
Genius 12080 points
Part Number: TDC1000
Other Parts Discussed in Thread: TDC7200, TS3A44159, TIDM-ULTRASONIC-FLOW-TDC

Hi team,

The customer uses TDC1000 +TDC-GP21 which is a similar TDC7200 device to design the flow meter. 

He We conducts a zero flow test at room temperature and pressure. The test result is in figure 1. The figure 2 is his original design.

Compare the two test results, the beating that is the Y-axis for the original design is about ± 2ns. But for the TDC1000 +TDC-GP21

test result, the beating  is about ± 4~5ns. Now the customer needs to reduce the zero flow value for the TDC1000 +TDC-GP21 design.

 
I found the SNIA020 document explained how to reduce the  zero flow value. In the 6 Measurement Accuracy Considerations of this

document, it explains we can match the two transducers impedance to reduce the  zero flow value. 

For the TS3A44159 circuit, I have drawn the TX circuit and the RX circuit. Please check the figure 3.

If COM pin is connected to NC pin for TS3A44159, the TX circuit and the RX circuit are shown in the figure 3. 

If COM pin is connected to NO pin for TS3A44159, the TX circuit is connected to XDC1 and the RX circuit is connected to XDC2.

So, we only change XDC1 and XDC2 through using TS3A44159 as I explained above and the capacitance and

the resistance are the same when the circuit is working. Then this can achieve the transducers impedance matching. 

Q1. Is my understanding correct?

Q2. But compare to the the TX circuit  and the the RX circuit, why are they different? 

For the TX circuit, there is a  200 ohm resistance  in parallel to the GND. Why is the 200 ohm resistance needed in parallel to the GND?

I also check the TI Design for   the flow meter.  TI Design :

From the schematic of this TI Design, I found the TX circuit and the RX circuit are the same that a 200 ohm  resistance

and a 300pF capacitance are in series. This is different from the figure 3 circuit. 

So, would you explain why the TX circuit and the RX circuit are different?

That is I would like to get the principle for the resistance and the capacitance.


                                                    Figure 1  TDC1000 +TDC-GP21

 


                                                             Figure 2 original design

 


                                                                     Figure 3

SNIA020:

http://www.ti.com/lit/an/snia020/snia020.pdf

Best Wishes,

Mickey Zhang

Asia Customer Support Center

Texas Instruments

 

 

  • 0
    TI__Mastermind 31655 points
    Hi Mickey,

    The circuit you have drawn in figure 3 is not correct. The resistor to ground is not in parallel with the transducer, since it is between TX and GND. If the resistor were in parallel with the transducer you would get a voltage divider that halves the output voltage of the TX pin. In its correct position, it provides a path to bleed down excess energy from the transducer to reduce decay time after the TX pin has stopped supplying voltage.

    The receiver circuit has the resistance to ground in parallel with the transducer since the transducer itself is supplying the voltage, but it provides the same purpose to allow a path for excess energy to drain to reduce the decay time for fast measurement cycles. The series capacitance on the receive circuit is the same that is in the TDC1000-TDC7200EVM and the TI design that you linked, and it is for setting the gain of the LNA inside the TDC1000.

    Regards,
  • 0 in reply to Scott Cummins
    TI__Genius 12080 points

    Hi Scott,

    Thanks for you explanation. 

    After I modify the correct circuit for the Figure 3, for the transducers impedance matching question,
    I also want to verify this as below:

    If COM pin is connected to NC pin for TS3A44159, the TX circuit is connected to XDC2 and the RX circuit is connected to XDC1.

    If COM pin is connected to NO pin for TS3A44159, the TX circuit is connected to XDC1 and the RX circuit is connected to XDC2.

    So, we only change the position of the XDC1 and XDC2 through using TS3A44159 and the capacitance and

    the resistance are the same when the circuit is working. Then this can achieve the transducers impedance matching.

    Since I do not understand the principle of the transducers impedance matching through  TS3A44159, 

    Would you like explain this in detail?

  • 0 in reply to Scott Cummins
    TI__Genius 12080 points
    Hi Scott,

    Would you help me verify the above question? The customer is waiting for my reply.
  • 0 in reply to Mickey Zhang
    TI__Mastermind 31655 points
    Hi Mickey,

    The switching does not only change which transducer is connected to the transmit or receive pins. The circuits between the RX and TX pins and the transducers are different. In my previous post I described the function of those circuits.

    Regards,