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TDC1000-GASEVM 40khz ?

Nicolasd
Prodigy 10 points

Hello,

I'm trying to measure air flow with TDC1000-GASEVM and a 40khz transducer.

I have 4 transducers :

2 x 400EP18A and 2 x A-18P20

Before measuring air flow, i tried to measure distance between two transducers.

I tried many configurations but i never had a clean COMP_IN signal. Does anyone succeed to use 40khz with TDC1000-GASEVM ? The signal seems to be completly filtered.

Generated pulses seems to be good and very clean 30vp-p :

Channel B TX signal in J5 and Channel A TX signal in J1

And now COMP_IN signal. Mode 1 or Mode 2 - PGA (0db) and LNA Active (same signal with capacitive or resistive)  A-18P20 are connected in J1 and J2 

No echo signal. I tried to change distance between transducers. No effect on COMP_IN. This signal is only noise.

If i change the PGA Gain, and LNA CTRL, the amplitude of the second part of the signal (juste after the pulses) change but it's impossible to see any echo.

Checked TDC1000-HV Driver EN2 change the same amplitude too.

I tested with J2 disconected from the transducer :

Channel B, transducer only connected on DSO. Distance about 8cm. COMP_IN is far to look like this. 

What can i do to have some results ?

My config tab :

  • 0
    TI__Mastermind 21685 points
    Nicolas,
    I will come back to you on this.
    Thanks,
    Vishy
  • 0 in reply to Vishy Viswanathan
    TI__Mastermind 21685 points

    a) The beam angle of both these transducers are very wide (85deg). With such wide beam angle, it's may be hard to see a good echo. You can check the bestarttech 200kHz transducer (beam angle 7deg) we have used here

    Please check this posting also (if you have not already)

    b) The 40kHz transducer has a very long ringdown time. In standard tof mode in addition to this ring down time of the transducer you have a 128 T0 cycles of common mode settling period (note C diagram from DS below)

    At 1.33MHz system clock this period is almost 100us. You also have 48us of autozero. So you can see target only beyond 148us tof. Hence it's better to use short tof mode (see picture from DS below) for your case and look for the echo

    Here common mode settling time and autozero are done before TX excitation. In Timeout register, you can select short time of flight. 

    c) Also, look at the EN1 period. Initially, better to keep it very long. At 40kHz excitation (24us per pulse), you need this period quite long. 

    d) Looking at the registers, I realize you are using Mode 1 and trying to see the echo on the second transducer. Comment (b) still applies for Mode 1 as well. In this case, you have to be careful about the spacing between the two transducers and make sure the second transducer can see properly reflected wave of the first transducer. If the beam angle is very wide as in this case, there can be quite a bit of cross talk. See

    www.ab.com/.../Sensing-Range-and-Effective-Beam.html 

    Thanks,

    Vishy