[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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