Ken Kundert
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Yes, you could very well be experiencing dielectric absorption from the board. This is a known phenomenon. I believe it is possible to upgrade your board material to reduce the problem. Or you could reduce the area of the trace that is exhibiting the dielectric absorption. Or you can eliminate the ground plane beneath the trace. Anything to reduce the capacitance.
As far as what time constant to expect, that is the thing about dielectric, there is a continuum of time constants from very low frequencies to very high frequencies. If you want to test for dielectric absorption, build a capacitor by etching parallel plates on your board material. Then charge the capacitor for a long time. Then short capacitor with a ammeter, preferably one that allows you to plot the current over time. The current through an ideal capacitor will be an impulse. Or, if the the ammeter has a resistance, then there with be an RC time constant. Any other time constants are associated with dielectric absorption. You will find that the remaining time constants will be hard to pin down because there is so many.
If you want to actually build a model for the absorptive capacitor, you should follow instructions in the paper.
-Ken
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