### Q

I have questions about the AD780 long term drift spec need your help.In the datasheet, the long term stability is +/-20ppm/1000hr (typical), and

there is note for it, says "The long-term stability specification is no

cumulative. The drift in subsequent 1,000 hour periods is significantly lower

than in the first 1,000 hour period."

My questions are:

1) what does the "subsequent 1,000 hour periods " mean in the datasheet. Does

it means, once the part worked cumulately more than 1000hr, the drift will be

lower(better) than before? Or if the part have worked more than 1000Hr, then we

power down it, then power up it again, does the stability specification could

go back? Hope I explain my question clearly :)

2) how do we test the long term stability? And is the +/-20ppm/1000hr spec

just a statistical?

### A

Long term drift refers to the warm up time and operation of the part. So once apart is powered up( in a static or dynamic way) and left alone, its output will

shift until it reaches its stability. The majority of the drift happens at the

early hours. it is usually expressed in ppm/1000 hours. Long term drift is a

"random walk" phenomenon and increases with the square root of the elapsed

time. There are 8766 hours/year. Therefore 1 year figure will be sqrt(8.766) ≈

3 times the 1000 hour figure. ten year value will be roughly 9 times the 1000

hour value. In practice, things are a little better even than this, as devices

tend to stabilize with age. We usually power up couple of parts in the lab in a

controlled environment and let it sit there for 1000 hrs and collect data at

different time intervals. AN-713 for more details.