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Re: Wide loop filter considered (not so) harmful

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  • Scotty
    Hi All, The input bias current in a PLL op amp will be the main determiner of PDF spurs. Trace, board cleanliness is usually secondary. If both were perfect,
    Message 1 of 17 , Aug 25, 2005
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      Hi All,
      The input bias current in a PLL op amp will be the main determiner
      of PDF spurs. Trace, board cleanliness is usually secondary. If both
      were perfect, there would be no PDF spurs. This is also assuming the
      loop capacitors have absolutely no leakage (yeah, right).
      I have used both of these amplifiers for the PLL 1, with these
      results: (The PDF is at 668 KHz)
      part BW Slew Rate Voffset InpBiasCur V Noise I Noise PDFspur
      OP27 8MHz 2.8V/µs 30µV 15nA 3.2nV/rtHz 1.7pA/vHz -31 dBc
      AD8671 10MHz 4V/µs 20µV 3nA 2.8nV/rtHz 0.3pA/vHz -54 dBc

      The AD8671 is an extremely close substitute for the OP27 and has good
      results for the PDF spur due to much lower Input Bias Current.

      Do not worry about the Input Voltage Offset. This is critical, only
      when designing fixed gain amplifiers. The PLL op amp is essentially
      open loop at DC and the op amp doesn't care about the offset voltage.

      The other two Op Amp specs to look for are lowest possible Input
      Voltage noise and Input Current noise. These would be important in
      very low frequency PLL's but not in a 1000 MHz PLL. The 20LogN noise
      is way over any noise the op amp creates.

      If you can find any op amp that beats the above specs, by all means,
      use it. Just make sure it will handle the high VCC voltage (+20v).
      Cheers, Scotty

      --- In spectrumanalyzer@yahoogroups.com, "swetterlin"
      <swetterlin@m...> wrote:
      > Hi John
      > Input bias current is the current in/out of the inputs; input offset
      > current is the difference between the Input bias current for the two
      > inputs. You are right, they are both whoppingly huge on the TLE2141.
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