LMF 100 - Mode 3 - design calculation

Discussion in 'General Electronics Chat' started by atferrari, Oct 28, 2011.

  1. atferrari

    Thread Starter AAC Fanatic!

    Jan 6, 2004
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    LMF100 switched capacitors filter.

    I intend to use it for an f0 =10 Hz thus fCLK to drive the filter (ratio 100:1) should be 10*100 =1000Hz but I have available only 120 Hz. The micro in charge cannot provide 1KHz what would make my life easier.

    To use it in Mode 3 as a LPF where I can adjust the fCKLK / f0 ratio via resistors R2 and R4, I calculate:

    Asuming R1 =18K and HOLP =1, R4 is also =18K.

    To have a relationship 100:1, I calculate R2 as per the equation

    fo = fCLK/100 * SQRT (R2/R4), obtaining

    R2 = 1250000 ohms for a ratio of 1000/120 =8.33333...

    My questions:

    a) Is it actually appropriate to use R1 =18K? How to find out that?

    b) R2 seems to be quite high (!) Is it convenient for this kind of circuit?

    c) To make R2 smaller I should make R1 also smaller. Is that convenient as well? If yes, how much less?

    Datasheet here:

    http://www.google.com.ar/url?sa=t&rct=j&q=lmf100%20datasheet&source=web&cd=1&ved=0CCMQFjAA&url=http%3A%2F%2Fwww.swarthmore.edu%2FNatSci%2Fecheeve1%2FRef%2FDataSheet%2FLMF100.pdf&ei=oumqTojPCcLq0gGXt7GGDw&usg=AFQjCNGEdtviaBKR6vBPoKNYZc9PBKlkng

    Help appreciated.
     
  2. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    If you use a clock of "only" 12 times the wanted frequency, you will end up with a lot of distortion.
    Here is a comparison of the 50 X and 100 X sampling modes from the datasheet:

    [​IMG]

    imagine what will happen with the 12 X sampling.

    You could make the clock frequency higher using a PLL with a divider like the 4046 with any counter.

    Bertus
     
  3. atferrari

    Thread Starter AAC Fanatic!

    Jan 6, 2004
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    In fact, mode 3 is supposed to overcome not having the exact clok available.

    The example of calculation given in the datasheet deals with exactly that.

    In my case I still intend to use no more nor less than a 100:1 relationship.
     
  4. crutschow

    Expert

    Mar 14, 2008
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    Why did you select R1=18k?

    The internal op amps can drive a load as low as 5K so you can have a feedback resistor as low as that. The corresponding input resistor can be as low as you are able to drive.
     
  5. atferrari

    Thread Starter AAC Fanatic!

    Jan 6, 2004
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    That was mi initial choice, close to the 20K selected in the example in the datasheet.

    Finally I tested everything with 6K8 for R1 and R4 thus R2 = 742K

    So far, it is not working. I am using +/- 5V supply with a clock from the micro (0 to 5V).

    Somewhat disapointed, but looking for eventual mistakes.

    Gracias Carl.
     
    Last edited: Oct 30, 2011
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