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LM311 V-F

Discussion in 'Electronic Design' started by AJ, Nov 2, 2006.

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  1. AJ

    AJ Guest

    HI,

    I am currently using a LM311 as a V-F. I have it set for 150mV/Hz with the
    following components.

    Rt = 200K
    Ct = 100nF
    Rs = 13.8K
    Cl = 1uF
    Rl = 100K

    For testing I have used some components that I have had laying around and I
    am finding that the output is greatly effected by temperature. The
    resistors are surface mount metal film but I have used an XR7 for Ct and a
    Tantalum for Cl. The datasheet recommends NP0 or C0G series for Ct and I
    have found one with 30ppm/C in an SMT package ok but it also says I should
    use a Mylar Cap for Cl. I have had a bit of a look around and the only 1uF
    Mylar caps I can find are huge and not available in SMT. Can anyone
    recommend a good choice for Cl? I was also wondering if Cl had to be 1uF, I
    am guessing it does but the datasheet isn't very clear, maybe I could use a
    smaller value?

    Best regards.


    AJ
     
  2. Eeyore

    Eeyore Guest

    1uF plastic film caps are quite large but mylar typically aren't the smallest.
    Mylar is just a DuPont trade name. Look for generic polyester film instead. A
    leaded cap is much more likely to be available at this value btw.

    Graham
     
  3. AJ wrote:


    The rate is determined by the ratio of the two capacitors, so they
    GOTTA be very stable.


    The actual capacitor values are not critical, it's only their ratio
    that is important.

    Just dont go much below say 500pf as then stray capacitances may become
    significant.


    So you could scale down the values by a factor of 100 easily. Maybe
    even 1000 if there isnt much stray capacitance.
     
  4. Eeyore

    Eeyore Guest

    In which case make them *both* polyester film as the tempco will at least track.

    Graham
     
  5. Eeyore

    Eeyore Guest

    Is that what it is ? Id never realyl thought much about it.

    There a very high dissipation factor with that type.

    Graham
     
  6. Mylar SMT capacitors are not available, because polyester cannot
    survive the high temperatures required for the SMD process.

    Here's one series that goes up to 1uF in SMT, with somewhat different
    characteristics because of the different dielectric:

    http://www.panasonic.com/industrial/components/pdf/ef038_ecpu_dne.pdf

    And some nice 5mm lead pitch through-hole stacked-film caps (molded
    case and dipped, respectively):

    http://www.epcos.com/inf/20/20/db/fc_05/MKT_B32520_29.pdf
    http://www.panasonic.com/industrial/components/pdf/abd0000ce8.pdf


    Best regards,
    Spehro Pefhany
     
  7. Eeyore

    Eeyore Guest

  8. Fred Bartoli

    Fred Bartoli Guest

    Eeyore a écrit :
    It'll be better, but saying they'll track is overstating.
    Caps' tempco depends on dielectric but also on the cap construction.
    For ex. try a stacked vs wound cap, with the same dielectric.
     
  9. Eeyore

    Eeyore Guest

    I had a couple of 'box polys' in mind.

    Graham
     
  10. Rich Grise

    Rich Grise Guest

    If you're going that small, wouldn't that put it into a range where you
    could use NP0s or C0Gs for both? Would they track better?

    Thanks,
    Rich
     
  11. AJ

    AJ Guest

    Thanks to all for the replies. I used some Polyester Caps and it fixed the
    temperature stability problem but im still finding that the output is not
    linear to the input for some reason. Rather than spending a lot of time
    with it I have decided to use a PIC12F510 instead. Its cheaper, comes in a
    surface mount package, requires few external components and can do
    everything the LM331 was meant to do for me with enough accuracy for my
    application.

    Best regards


    AJ
     
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