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PGA460: Decay time is not consistent in different boards

Pablo Martinez80
Intellectual 275 points
Part Number: PGA460

Hi,

We are facing some issues with our latest product. It seems that the decay time is not consistent between different pcbs and it is not very similar to the one we see on the PGA460 evaluation board BOOSTXL.

We have done hundred of tests. I will try to summarize in a few lines the issue we have so maybe you can shed some light here.

Until now we have produced around 30K pcbs and all of them behave similar. but now we created a new product (Alll schematics related to PGA460 are the same as the previous one, no changes here) And some of the pcbs (mounted by our electronical engineer not a mounting factory) show the following pattern on the decay time:

If the pcb is good, the decay time starts from the top of the chart as shown in a very few of the curves. 

After replacing the transformer, some of the pcbs showed a better behavior but other did not.

Then, we tried comparing the signal from the TEST pin of the PGA460 (TEST_MUX register set to 0x20) and the signal of the transduder and we see this on our pcbs (an almost good pcb as the issue is very small):

But, repeating this test on the PGA460 evaluation board BOOSTXL, shows the following:

Whis is a bit longer than expected.

If we compare 2 curves from the pga from our pcb and the PGA460 eval board (Focused only on the decay time, not the echo response):

The evaluation board decay time is a bit longer and we do not see any issue on the top of it.

We have not seen any wrong behavior on the sensor response. Echo seems to be ok, peak and wide too and thermal chamber showed the same. But we are not fully satisfied as we do not know the root cause for this issue.

Schematics are the same as shown on the reference manual. 1 transducer (Murata MA58MF14-7N).

I do not know if we could improve this behavior by configuration. Is it possible that some of the registers could modify it? I do not know, for example DEADTIME (We do not know what this register does).

If you need more information, we can provide it.

  • 0
    TI__Mastermind 23045 points

    Hey Pablo,

    Thanks for posting to the PS forum!

    I would imagine the decay to vary slightly between the same type of PCBs, due to component tolerances being different from board to board (capacitors, resistors, inductors, and even transducers). But I would definitely expect to see a difference between 2 different PCBs mainly due to various parasitic effects that having a different layout can introduce. 

    In the scope captures shown, it looks like the BOOSTXL EVM had a longer decay time than in your board by ~100us, but was this test taken using the same voltage, the same amount of pulses, etc? The amount of pulses transmitted by the devices should be equal, if you are transmitting more pulses then you will be saturating the input for a longer period of time (since the TX and RX share the same path). And of course this would be configurable via registers.

    DEADTIME is used if you are using the device in direct drive mode to drive an external MOSFET. What the DEADTIME does in insert an additional amount of time between exciting the highside external FET and low side internal FET. Reason being is that if both low side and high side FETs are on at the same time this could cause a shoot-through condition which could damage the FETs and the device itself.

    Looking forward to your response!

    Best,

    Isaac

  • 0 in reply to Isaac Lara
    Intellectual 275 points

    Hi Isaac Lara. Thank you for answering me.

    Both board (EVB and cuscom pcb) are using the same profile for the PGA460. That is why we do think it is quite strange the response.

    Do you have a "best layout" design for the PGA460? It would be good if we could have it as reference in case our design is causing this issue.

  • 0 in reply to Pablo Martinez80
    TI__Mastermind 23045 points

    Hey Pablo,

    Thanks for confirming that the same profile was used in both of these. In reality, I do not see a problem with the echo profiles between your board and the PGA460 board, in fact most users are usually concerned with trying to reduce their modules decay time, so the fact that your system has a smaller decay time than the EVM it means that you have a better layout than the one found in the EVM. A faster decay typically means that your system is more free of the parasitic components I mentioned in my last reply. Which could make sense since the EVM is built in separate modules so this does not really help creating the most ideal layout. Things like the separate transducer module from the main board in the BOOSTXL-PGA460EVM, could introduce some differences that are most likely non-existent in your module.

    I believe that trying to chase down the same echo saturation profile as the EVM is probably not the best issue to pursue, especially since a lower decay time typically indicates a superior layout.

    I think the real question would be if you are seeing a large variance from custom board to custom board, but looking at the data you provided it seems like the echo decay seems fairly consistent. If you are trying to alter your decay profile, I would image component matching would be your best way to achieve this. You can also take a look at what effect changing the transformers tuning capacitor and damping resistor have in your system.

    You can read about these components in section 4.4.2 and 4.4.3 of this app note: https://www.ti.com/lit/an/slaa732a/slaa732a.pdf

    Best,

    Isaac

  • 0 in reply to Isaac Lara
    Intellectual 275 points

    Hi Isaac,

    We are not trying to achieve the same decay time as the EVB. What we do not understand is why we have to large differences between our pcbs (same design, same components, the same sensor unit tested in all of them). We are worried about the strange shape of the decay time:

  • 0 in reply to Pablo Martinez80
    TI__Mastermind 23045 points

    Hello Pablo,

    Thanks for the clarification. As far as the odd waveform this is usually caused by a really early reflection of an ultrasonic signal, an early reflection typically caused by a nearby object or perhaps the housing of the transducer. I have attached images below for comparison.

    Here is a capture without an early obstacle:

    Here is a capture with an early obstacle:

    I hope this helps!

    Best,

    Isaac