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TUSS4470: Signal envelope is too extended

Dmitry Egorov
Prodigy 100 points
Part Number: TUSS4470


Tool/software:

Hello.  

Here is the Vout from TUSS4470 (yellow) and the input signal from the piezo (blue)  on the same screen. As can be seen, for some reason the yellow envelope is about twice in length vs the actual signal.

I tried to reduce the Cflt hoping it will reduce the time constant of the log amp, but reduction from 4.7nf (I work at 130kHz) down to 680pf did not help much. 

I have first and last log amp stages disabled, BPF enabled at 135kHz.   

Are there any ideas why this happens and is there a way to fix this? I'd like the envelope to be an envelope and not this overextended crap...

Thanks!

  • 0
    TI__Genius 14320 points

    Hi Dmitry,

    Thank you for posting to the Sensors forum!

    I know you already decreased Cflt quite a bit already, but could you try decreasing Cflt down to 60~70pF to see if that makes a difference?

    Best,

    ~Alicia

  • 0 in reply to Alicia Rosenberger
    Prodigy 100 points

    Hey Alica

    Thank you for your recommendation. 

    I've just tried Cflt = 68pf - it did not help much, but the signal becomes way too noisy. Brought it back up to 680 pf. 

    TUSS4470 is a mysterious IC, makes me wonder if it's internals really match the datasheet block diagram. I am wondering if it's log amp has some peak detector circuit with a slow discharge path?  It would be nice to know the actual time constant or decay rate of the log amp. 

    What helps a little better to improve the envelope discharge is switching the input filter from BPF to HPF with a cutoff of 100kHz.  It reduces an output signal amplitude though. Which is somewhat strange, as I work at 130 kHz.

     

    I am still looking for some ideas on how to make an envelope discharge faster for it to look like a glove...as I use digital matched filtering so overextended baggy envelope reduces the resolution of the radar.   Thank you again !

  • 0 in reply to Dmitry Egorov
    TI__Genius 14320 points

    Hi Dmitry,

    I am sorry that reducing Cflt didn't seem to help much, I will need to look into this further to see if there are any other ways to reduce the envelope length.

    Thank you for your patience.

    Best,

    ~Alicia

  • 0 in reply to Dmitry Egorov
    TI__Genius 14320 points

    Hi Dmitry,

    I am sorry that reducing Cflt didn't seem to help much, I will need to look into this further to see if there are any other ways to reduce the envelope length.

    Thank you for your patience.

    Best,

    ~Alicia

  • 0 in reply to Alicia Rosenberger
    Prodigy 100 points

    Thank you Alicia, I am looking forward to getting your advice !

  • 0 in reply to Dmitry Egorov
    TI__Genius 14320 points

    Hi Dmitry,

    Apologies for the delayed response and the inconvenience. After speaking to the other members of my team, unfortunately there does not seem to be any other ways that we can think of to reduce the length of the envelope to better fit the signal from the piezo.

     Best,

    ~Alicia