Poul-Henning wrote:
The input signal to the integrator is continuous and jump-free and the relevant time constant is sub-second. Dielectric absorption doesn't matter when there are no voltage jumps.
That is a very common misconception, probably fostered by the usual test methodology for DA. But it is not correct.
DA is a charge-migration phenomenon, so it occurs *whenever the charge on a capacitor changes.* A step function in the charge very obviously represents such a change (and for this reason is used for testing the parameter in most circumstances), but so do gradual changes in the driving charge. These can be less obvious (than step changes), depending on the time scale of the change relative to the time scale of the DA process in the particular capacitor.
There are several dimensions to DA -- not just how much charge isn't returned immediately, but what the "tail" looks like (i.e., the time profile of charge return, generally measured as current vs. time). Different dielectrics (and, to a certain extent, different construction techniques) exhibit different charge-return profiles.
Long-TC integrators do tend to mask the effects of DA, because the time scale of changes in the driving charge is closer to the time scale of the delayed charge return caused by DA. But that does not mean that DA isn't important, because slow integrators are frequently used where extremely high precision is required (such as integrators for high-resolution DACs and EFC servos for precision oscillators), so the tolerable amount of delayed charge return is extremely low.
As with any circuit, the A9 integrator is "good enough" when the errors it causes are swamped by other accumulated errors. But I'm no fan of "just good enough" design, particularly in a case like the 6065A where time nuts are diligently working to improve the performance of the instrument. Some times, "just because it's better" is a sufficient reason to overdesign, particularly where the incremental cost is low and especially where the projected number of units is low, both of which are true WRT the improved A9 board.
Best regards, Charles _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
