Analog Devices AN-713 Application Notes

AN-713

TIME (Hours)

LONG-TERM DRIFT (ppm)

200

–150

–100

–50

0

50

100

150

0 100 200 300 400 500 600 700 800 900 1,000

APPLICATION NOTE

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The Effect of Long-Term Drift on Voltage References

by Colm Brazil

Long-Term Drift (LTD) in voltage references is a param­eter that can be confusing and misleading. It is given as
a typical parameter in data sheets, but can cause sig­nicant accuracy limitations in system. Unlike tempco
and initial accuracy, where a one-time calibration can be
used to eliminate these errors, attempts to reduce LTD
requires frequent system calibration. This can be both
time consuming and expensive. Therefore, it is important
to understand the signicance of LTD.

Most data sheets specify LTD as ppm/1,0 0 0 hour s.
1,0 0 0 hours is equal to 41.5 days, and in one year there
are 8,766 hours. Therefore, if a part is specied to change
by 70 ppm/1,000 hours, does this mean that in one year it
will change by 613 ppm? No, it does not. This is a common
mistake that is made by only reading the data sheet but
not understanding what LTD really is.

The following graph shows a typical LTD curve of a typical
ADI band gap reference. The output voltage of this part
has been recorded every hour for 1,000 hours in a controlled
environment with a temperature of 50C.

As the graph shows, in the rst 200 hours there is a lot of
movement in the part. After this initial movement there
is very little change in the V
Over the next 1,000 hours the variation is normally less
than one quarter of that in the rst 1,000 hours. This
trend continues with the change being an approximate
cut in four from the previous time period. The changes
in output take a “drunken walk” pattern: the changes are
random and cannot be predicted, so the output might
change positive in the rst time period but negative in
the second.

• 70 ppm/1,000 hours does not mean the part will change
by 613 over one year but will change more in the area
of 140 ppm/year

• The changes are random, so if the part starts off in one di­rection it does not mean it will continue in that direction

Ways to Eliminate LTD Effects

• Calibrate the system after 200 hours, thus eliminating
the early changes

• Burn in the part for a short period of time to eliminate
the early changes

for the last 800 hours.

OUT

Figure 1. Typical Long-Term Drift Over 1,000 Hours

REV. 0

AN04698–0–2/04(0)

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