[time-nuts] V standards

Mike Monett XDE-L2G3 at myamail.com
Mon Dec 1 10:41:10 UTC 2008


  >Mike

  > In testing  high resolution DACs (20+ bits) used in  GPSDOs  etc a
  > high resolution DVM (or equivalent) with low noise and  good short
  > term stability is useful if not essential.

  > Monotonicity and  perhaps linearity together with good  short term
  > stability are generally more important than absolute accuracy.

  > Higher resolution is usually accompanied with higher accuracy.

  > DVMs like the 3457A. 3456A etc dont have sufficiently low noise or
  > good enough  short  term stability for testing  DACs  with  24 bit
  > resolution.

  >Bruce

  I wonder if 20+ bits is even realistic for a frequency reference.

  That would  give a LSB of 1/2^20=9.53e-7, or 1ppm, and  none  of the
  voltage references  discussed will maintain this accuracy  over long
  periods.

  And what would a 24-bit DAC be used for? From  previous discussions,
  high accuracy sources, like H-Masers are not adjusted. And  it would
  seem silly to put such a high resolution DAC on a OCXO.  That leaves
  Cesium, which  I understand are used in GPS satellites  and  do need
  adjustment, but  I  don't   have   any   information  on  the tuning
  sensitivity to figure the effect 1 LSB would have on the frequency.

  I haven't had much luck finding a true 24-bit DAC. There  are plenty
  of stereo dacs, but they can have gain drifts of 100ppm/C,  which is
  useless for a reference.

  With a  5V reference, a 24-bit DAC would give a LSB  of 5e9/2^24=298
  nanovolts. If  I had to test one, and didn't have a  3458A,  I could
  use a 3456A. It has a resolution of 100nV on the 100mV range,  so it
  could verify the bottom portion of the DAC from zero to  100mV. Once
  the lower  8 bits are confirmed good, the rest of the  DAC  could be
  checked by  exercising each high-order bit singly,  then  in various
  combinations with the other bits.

  Another method  would  be  to use two 24-bit DACs  and  a  AD8571 to
  measure the  difference between them. The AD8571 could be  set  to a
  gain of  100,  so  298nV becomes 29.8uV  which  is  well  within the
  capability of a 3456A.

  The test  would  be  to  set  both  DACs  to  zero  and  measure the
  difference in output voltage. It should be close to zero.

  Next, set  the LSB of the reference DAC to 1. The 3456A  should read
  close to 29.8uV.

  Then set  the LSB of the test DAC to 1. The 3456A should  read close
  to zero.

  Follow this  procedure  with   each   bit   in  turn  to  verify the
  functionality, then  test various combinations to check  for  two or
  more bits  that  are  stuck  together.  The  LSB  could  be  used in
  conjunction with  the  bit being tested  to  prevent  saturating the
  AD8571.

  The above tests are not as good as a dedicated test for  a precision
  DAC, but might serve in lieu of spending $4k to $7k for a 3568A.

  Regards,

  Mike Monett




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