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PGA460: Distance compensation for different temperatures. How to use DECPL?

Jaatje
Prodigy 185 points
Part Number: PGA460


The datasheet (pga460-q1.pdf) states that the temperature decoupling can compensate the non-linearity of the transducer. However, to use the DECPL function, the capacitor has to be tuned for this function to work. How is the capacitor (Ct) determined for this function? Neither both reference designs explain how they came to the Ct value.

Thanks in advance!

  • 0
    TI__Mastermind 27950 points

    Jaatje,

    The temperature decouple feature is exclusively dependent on the transducer you use, and the ambient temperature you intend to operate the transducer in. Also note this is only required for system that use a transformer driver approach. Added a temperature compensation capacitor will have no effect in a direct-drive solution. That is because the capacitor is compensating the change in the transducer's electrical characteristics so that the transformer's secondary can continued to be matched in center frequency. Transducer matching is discussed in the PGA460 Ultrasonic Module Hardware and Software Optimization app note.

    Some transducers are designed to operate consistently across a large temperature range, and may not require any external capacitor compensation. Ultimately, this effort will require you to characterize the equivalent BVD model of your transducer across temperature, but more importantly make a performance comparison across your required operating range.

    For example, if you need to be able to detect your worst case object from 30 to 500cm in a temperature range from -40 to 125°C. You should spot check time-of-flight ranging performance around your minimum distance (30-50cm) and around your maximum distance (4-5m). In many cases, the transducer performance will degrade at very high or low temperatures, such that the decay time extends, pushing your new minimum detectable to 40cm, or result in a loss of SNR at long range, such that you can now only detect upwards of 4m reliably. As far as I know, there are no tools to help model both the transducer's BVD model and performance across temperature, so it is a manual tuning effort if operating in extreme temperatures is critical to your application.

    If you don't need this feature, I suggest you do not implement it, save yourself from having to add another FET to your board, and keep your DECPL pin floating.