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BOOSTXL-TUSS4470: BOOSTXL-TUSS4470 and MSP-EXP430F5529LP - Large noise when working with external power supply

Itamar Eliakim
Prodigy 110 points
Part Number: BOOSTXL-TUSS4470

Hi,

I am trying to evaluate distance over a large pool(0 - 7m) using the BoostXL-TUSS4470 and 1MHz transducer.

When working with internal USB 5V everything works properly, when connecting the external power supply at 20 - 36V I keep getting this noise on the baseline.

What causes this noise and how it can be filtered to achieve the same results as the internal USB 5V one?

I read the note that J6 Pins 3-4 should be removed to reduce this noise, results are the same.

This was taken at the pool at 1m distance:

Thanks,

Itamar

  • 0
    TI__Mastermind 27780 points

    Hi Itamar,

    When using an external supply for the VPWR of the TUSS44x0, you should have the jumper configured as shown in the "Standard" power mode on Table 1 of the TUSS44x0 EVM for Ultrasonic Sensors User's Guide (Rev. A). This means only J6 pins 3-4 should be connected. The J6 pin 3-4 short connects the TUSS44x0 ground (GND) to the MCU ground (GLP), which is always required. If you do not short these two pins, then you must make GND and GLP common at the external power supply ground. This is what is meant by "reducing noise"; the grounds being common at the supply rather than the board typically helps reduce some of the transient noise caused by the SPI communication and other digital noise. In your case, I would just keep the grounds common at the board for simplicity since it was already working for you in the USB mode. The fact that you currently have the J6 pin3-4 shunt removed may explain why the noise is high.

    Ultimately, the stability of the noise floor is determined by the VDD rail of the TUSS44x0. The VDD should always be 3.3V when using the EVM-GUI. The VDD rail is generated by an on-board LDO when the J8 jumpers are installed, so unless you are also forcing 3.3V externally, VDD should not change between the USB and external power modes. To verify, probe VDD in the USB mode and then in the external supply mode to determine if VDD becomes less stable in your external supply mode. If the externaly power supply is noisy, it could be possible that it is causing the VDD and analog ground of the TUSS44x0 to become less stable. One solution to this is to add a low pass RC filter at the output of the external power supply. This will ensure no high frequency noise is coupling onto the main power path.

  • 0 in reply to Akeem Whitehead
    Prodigy 110 points

    Thank you, Akeem.

    This noise also occurs when GLP and GND are connected.

    I have tested it with both power supply and external battery - results are the same, I do not think that this is related to the noise of the power supply.

    I will check the VDD voltage to confirm that this is the same on USB and external mode.

    After some more research, it seems that it is related to the capacitors CFLT and CINN.

    The same values that apply for small scale water tanks are completely different than the ones apply for large pools - is there any way to overcome this issue?

  • 0 in reply to Itamar Eliakim
    Prodigy 110 points

    Hi ,

    Do you have any idea what can cause this noise? 

    The same values that apply for small scale water tanks are completely different than the ones apply for large pools - is there any way to overcome this issue?

  • 0 in reply to Itamar Eliakim
    TI__Mastermind 27780 points

    Hi Itamar,

    Were you able to confirm that the VDD voltage in the case of the external power supply is clean/stable? The 3.3V voltage should not contain ripple when probed.

    Regarding the CFLT and CINN values, these values are exclusively dependent on transducer frequency, and should not change because of the tank size. What CFLT and CINN capacitor values are you using for the small versus large tanks?

  • 0 in reply to Akeem Whitehead
    Prodigy 110 points

    Hi Akeem,

    I just purchased a smaller pool and somehow I cannot reproduce this issue at the smaller pool, I will get back to the testing on the larger one within the next week or two.

    I have some issues with the 1MHz at the smaller pool, these are the measurements using 1MHz transducer where the Shunts are located at C1 and C12 (which suits the requirements of 1MHz from the lookup table):

    These are the measurements from with SHUNTS located at C3 and C14:

    Same frequency but as you can see the C3 and C14 looks much better.

    Another issue is when continuously sampling the sensor, I keep getting this random noise:

    The amplitude of the two faulty detections are changed randomly, do you have any idea what causes this issue?

  • 0 in reply to Itamar Eliakim
    TI__Mastermind 27780 points

    Hi Itamar,

    The performance differences in the shunts is due to the difference in the amount of high pass filtering. In your second plot, the HPF is more aggressive, so the overall signal appears cleaner, but you'll notice the peak amplitude of your actual echo is also weaker, so there are tradeoffs to consider. If you are satisfied with the results of your C3 and C14 shunt configuration, there is no issue in proceeding with this option.

    Regarding the random noise, it could be:

    • a result of the SPI communication taking place while the time of flight measurement is taking place
    • the VDRV charge cap recharging over the course of the time of flight measurement
    • in-band noise from another source

    The best way to solve this is to probe the SPI SCLK pin, the VDRV pin, and VDD to see if the noise appears on any of these other pins also. If so, the noise from one of these pins is coupling onto the AFE receiver path.

  • 0 in reply to Akeem Whitehead
    Prodigy 110 points

    Thank you .

    I will check that and post the results in a few days.