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I am currently designing an ultrasonic sensor unit. One of the requirements is that the ultrasonic sensors located in the hazardous area need to be intrinsically safe. This requires placing a Zener barrier between the sensors and the control system.

I have tested two different types of ultrasonic sensors:

  • A standard ultrasonic transducer (40 kHz)
  • An integrated piezoelectric micromachined ultrasonic transducer (PMUT) with a system-on-chip (SoC)

For the standard transducer, I used a Texas Instruments evaluation board (BOOSTXL-TUSS4470) with an Arduino Uno. For the integrated sensor (CH101), I used an Arduino Zero along with the Arduino library for the sensor. Both setups worked fine without the Zener barrier, the CH101 continued to work even with the Zener barrier in place.

When I added the barrier between the standard transducer and the evaluation board, I could no longer see the echo on the oscilloscope.

  • Transducer: UTR-1440K-TT-R, 40 kHz, 1800 pF
  • Resistor: 80 Ohm
  • Damping resistor: 10 kOhm
  • Zener (2EZ14D5-TP): 14 V, transducer is pulsed with 12 V
  • Fuse (C308F-V-40MA-TR1): 40 mA

Diagram of the circuit

Although we are now using the CH101 for this project and the transducers are no longer needed, I am curious about why the barrier affected the echo so much. My assumption is that the capacitance of the Zener diode might have filtered out the echo, but I am not entirely sure.

Does anybody know the exact reason for this?

Scope image of the echo (Pink is the VOUT pin from the BOOSTXL-TUSS4470 and yellow is the positive terminal of the transducer) without the barrier and an object located 50 cm from the transducer:

scope image without barrier

Scope image of the echo (Pink is the VOUT pin from the BOOSTXL-TUSS4470 and yellow is the positive terminal of the transducer) with the barrier and an object located 50 cm from the transducer:

Scope image with barrier

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    \$\begingroup\$ Please show waveforms where the 0 volt line is in the same picture. Please ad a schematic of the circuitry either side of the zener barrier and, of the zener barrier itself. \$\endgroup\$ Commented Oct 22, 2024 at 14:26
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    \$\begingroup\$ What is the part number for the Zener diode? What is the capacitance of the piezoelectric transducer? Placing a capacitance C0 across a transducer with capacitance CT will act as a voltage divider, reducing your output voltage by CT / (CT + C0). \$\endgroup\$ Commented Oct 22, 2024 at 15:39
  • \$\begingroup\$ Hi @Andyaka, thanks for the feedback, I edited the post so all the info is now shown. \$\endgroup\$ Commented Oct 23, 2024 at 7:45
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    \$\begingroup\$ I suspect that the output that drives the transducer comes from a transformer. Try looking at the transducer waveforms. \$\endgroup\$ Commented Nov 1, 2024 at 12:02
  • \$\begingroup\$ @Djumbonia I have added my answer with expanded explanation, it would be interesting to see you confirm by practical measurements whether my assumptions are correct. \$\endgroup\$ Commented Nov 3, 2024 at 7:42

2 Answers 2

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A picture is worth a thousand words.
If we look at a zoomed-in portion of your oscilloscope captures, some details are almost self-revealing.

Here is the capture WITHOUT zener diodes. enter image description here

Notice 3 things:

  1. Output peak-to-peak voltage value is higher, at 13.40V.
  2. There are also noticeable NEGATIVE peaks on CH1.
  3. The Vout echo signal is fairly "clean" (almost no HF content).

Now we look at the captured signals WITH the zener diodes in place:
enter image description here

Now the same 3 things in the same order as above:

  1. Output peak-to-peak voltage value is LOWER, at 11.60V.
  2. There are HARDLY noticeable negative peaks on CH1.
  3. The Vout echo signal is not "clean" anymore; it contains the high-frequency content from the drive output.

Based on the above observations and the datasheet information about this circuit, this is what I conclude:

  1. The zener diode capacitance doesn't play much role in this scenario.
  2. The zener diodes in their forward direction are shorting out the negative peaks which we see in the first but not in the second capture. Try adding another zener diode of the same value to each of the two zeners BACK-TO-BACK so that both outputs can swing 14V in BOTH directions.
  3. Zener diodes typically start conducting at voltages lower than the specified zener voltage, and this is more noticeable in higher impedance circuits because they are more affected by low currents.
    If you want to allow for peaks of 14V in a relatively high-impedance circuit, you need to use zener diodes rated for at least 15V and up to 18V.
  4. From what I can see here, the high-frequency peaks at Vout could be caused by ground loop currents caused by the forward-biased zeners.
    I don't think that those HF bursts are actually there on the Vout, I think they are picked up through the ground because of the currents through forward-biased zeners, and you can verify this by connecting only one probe to Vout, with the probe's ground connected with a ground spring close to Vout ground.
    Since Vout is actually low-pass filtered, there shouldn't be HF content on it.
  5. Finally, the reason your echo output signal at Vout is greatly reduced is due to the logarithmic nature of its pre-amplifier: the lower the signal, the greater the amplification, and for the same reason there is a greater reduction in output voltage for a relatively small change on its input.
    It could also be that the ground loop currents caused by forward-biased zener diodes are reducing/cancelling a part of the signal which the pre-amplifier input sees.
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Your selected zener does not have a specified capacitance. That means it's almost certainly a "reasonably high" value, or else they'd brag about low capacitance. That is probably your issue. Also, your zener will draw 0.5uA at 10.6V, and presumably considerably more at 12V. The zeners I'm used to are generally specified at around ~1uA AT the specified working voltage, not some arbitrary voltage a little bit below it.

Excessive capacitance could filter out your receive signal; seems less likely, but could also affect the transmitted signal. I didn't find a current capability, output impedance, or other such spec. From your scope shots, I can't tell if the transmitted signal has been attenuated.

I will guess it is the capacitance, because your 10k "damping resistor" would draw 12/10000 = 1.2mA. This is much greater than the 0.5uA at 10.6V, but again we don't know how much current your zener draws at 12V. You could measure this with a DMM.

Try selecting a higher voltage zener, if your safety standards allow. Preferably, get one that has a specified low capacitance, and little leakage at 12V. For instance, littelfuse has SMF series which should have capacitance <1nF, albeit at a lower power rating. In general, you need a bit more headroom than 2V -- if you're driving at 12V you want a zener that won't clamp til 16-18V or so, depending on how much leakage you can handle. Of course, consult the datasheets.

Finally, I question your premise that you added zeners to both transducers and had it degrade the TI eval board but not the CH101. The CH101 has the transducer internal, so where exactly did you put the zeners? Not across the transducer, as far as I can tell.

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