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Log amps? for dB metering.

Discussion in 'Electronic Design' started by Guest, Oct 31, 2006.

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  1. Guest

    Guest Guest

    I'm trying to come up with an LCD dBv meter. Not the bargraph type but a
    numeric LCD type. It needs to have a range of +30 down to -70. 100 dB total range.
    the reference is .775V = "0" dBv. I want to use one of the PIC's for the controller.
    My question is should I use a Log amp to do the log calculations or think about doing it
    in software. The smaller pic's I've looked at don't offer much math capability.
    To do the calculations in hardware I think I'll need an RMS to DC converter,
    and a LOG amp. Then I would read the output of the LOG amp into the A/D converter in the PIC.
    Any Ideas?

    Thanks
     
  2. Mark

    Mark Guest

    Is this for audio or RF signals?

    For RF I would look a successive detection chips that convert AC to log
    DC. They might be usable for audio too.

    Mark
     
  3. Guest

    Guest Guest

    Thanks Mark,
    This is for audio up to 200Khz
     
  4. linnix

    linnix Guest

    You might have trouble doing A2D at this rate with PIC, not to menson
    the calculations.
     
  5. Ban

    Ban Guest

    I'm trying to come up with an LCD dBv meter. Not the bargraph type but a
    numeric LCD type. It needs to have a range of +30 down to -70. 100 dB total
    range.
    the reference is .775V = "0" dBv. I want to use one of the PIC's for the
    controller.
    My question is should I use a Log amp to do the log calculations or think
    about doing it
    in software. The smaller pic's I've looked at don't offer much math
    capability.
    To do the calculations in hardware I think I'll need an RMS to DC converter,
    and a LOG amp. Then I would read the output of the LOG amp into the A/D
    converter in the PIC.
    Any Ideas?


    You can do all of the above with a single IC: THAT4103, Design Note 119
    describes a 120dB range dB Meter.
    And please turn off that HTML in your posts to NG.
     
  6. Phil Allison

    Phil Allison Guest

  7. Eeyore

    Eeyore Guest

     
  8. Guest

    Guest Guest

    Thanks all
    Sorry about the HTML
    RonL
     
  9. Bob Eld

    Bob Eld Guest

    I'm trying to come up with an LCD dBv meter. Not the bargraph type but a
    numeric LCD type. It needs to have a range of +30 down to -70. 100 dB total
    range.
    the reference is .775V = "0" dBv. I want to use one of the PIC's for the
    controller.
    My question is should I use a Log amp to do the log calculations or think
    about doing it
    in software. The smaller pic's I've looked at don't offer much math
    capability.
    To do the calculations in hardware I think I'll need an RMS to DC converter,
    and a LOG amp. Then I would read the output of the LOG amp into the A/D
    converter in the PIC.
    Any Ideas?

    Thanks

    A 100dB range is a tall order. While you can use a PIC for the display, you
    can't use a PIC for the A to D function because these are only 10 bit
    coverters, which at best would give a range of 60dB. And that uses every
    bit! A log amp is also going to have problems with a 100dB range. Therefore
    you need to re-evaluate what you are trying to do. You could use a PIC with
    its built in A to D function for anything less than a 60dB range. Instead of
    a log amp, you simply digitize at the fastest rate the rectified peaks of
    the audio waveform then in software use a lookup table to convert the
    digital values into their dB equivalent which you display. No math is
    required. It's easy to add the necessary delays, storage and averaging so
    that the display is readable without flicker. The display drivers can all be
    in the PIC as well. LCD displays require AC drive which you need to become
    familiar with if you do it in software. A two digit display could easily be
    handled by an 8 bit pick. It would be possible to extend the range maybe to
    100 dB by using a switchable "front end" that simply amplifies low level
    signals by say 100 times (40dB) when the processor says it doesn't have
    sufficient signal. Forty dB is then subtracted from the display in software
    to make the display show the lower values.

    If you are serious about a 100dB range directly without switching, you will
    need to process at least 17 bits. You can use a 20bit A to D converter then
    a 20 bit or 24 bit or even 32 bit processor or DSP chip to do the math and
    display. That's a much bigger deal than the 60dB PIC single chip solution.
    Bob
     
  10. Guest

    That's assuming you do it the hard way; if you feed the signal to an
    AGC
    amplifier, the gain control pin of the amplifier can easily give you
    the
    dBv indication with eight bits of digitization. You just have to
    remember the gain of a transistor is logarithmic in base voltage,
    and you do the amplitude detection/logarithm function before
    digitization.
     
  11. Eeyore

    Eeyore Guest

    Reliably over 10 decades ?

    Graham
     
  12. linnix

    linnix Guest

    And the noise problems will be far worse than using a 16 bits A2D.
     
  13. Bob Eld

    Bob Eld Guest

    So, in your mind that is simpler than a switched gain, op-amp based front
    end using a few resistors, a FET switch, an op-amp and an 8 bit processor?
    How about cost, temperature stability, drift, offset and all of the other
    problems of using p-n junctions as circuit elements? Thanks, but I would
    stay away from analog p-n juction based designs and go as digital as
    possible if I were doing it.
     
  14. Ban

    Ban Guest

    Not only just lazy, but I'm on dial-up and my daughter is blocking the
    phone. I have the nice Applications handbook from THAT and I gave detailed
    information that even a complete idiot like Phyllis could immediately find
    the links. Well done!
     
  15. Phil Allison

    Phil Allison Guest

    "Ban" = Lazy Wog **** "


    ** Are you so full of shit cos your pet goat is blocking the dunny ???


    ** Just try using "THAT 4301" or "THAT" as inputs on Google - see how
    far you get.

    You arrogant pile of stinking wog shit !!






    ......... Phil
     
  16. Ban

    Ban Guest

    I gave in "that 4301"
    1st hit: http://www.thatcorp.com/4301desc.html
    2nd hit http://www.thatcorp.com/datashts/4301demo.pdf

    and I also tried without the space, same result.
    when clicking the first link, there is the "applications" link on that
    product page, which nicely lists AN119.

    You are even worse an idiot than I thought
     
  17. Phil Allison

    Phil Allison Guest

    "Ban" = Lying Wog **** "


    ** Shame how your FUCKWIT post gave the WRONG number !!!


    Quote:

    " You can do all of the above with a single IC: THAT4103, Design Note ...
    "


    Google responds to " THAT4103 " with ** ZERO ** matches.

    http://www.google.com.au/search?hl=en&q=that4103+++&btnG=Google+Search&meta=


    YOU PITA ASININE DAGO F U C K H E A D !!!!!!!!!!!





    ......... Phil
     
  18. Aren't their jellybean RSS circuits that routinely do this?



    --
    Many thanks,

    Don Lancaster voice phone: (928)428-4073
    Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
    rss: http://www.tinaja.com/whtnu.xml email:

    Please visit my GURU's LAIR web site at http://www.tinaja.com
     
  19. Guest

    Guest Guest

    Thanks all
    Analog devices AD636 will work for this project. I'll use a preamp for the
    low level signals
    and a PIC for the LCD and switching gains.

    Thanks
    RonL
     
  20. Zak

    Zak Guest

    I'd use 2 x the PIC solution. One with a preamp. The PICs can figure out
    which of the 2 has valid data on the output. i.e. the range switching is
    digital.


    Thomas
     
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