Thanks Charles. That makes sense, but at the expense of adding unwanted complexity. As I've been moving the setpoint around this morning, I think I see a way to characterize what it's doing. Maybe I can come up with a small correction table or formula that's good enough for my purposes.
Bob >________________________________ > From: Charles Steinmetz <csteinm...@yandex.com> >To: Discussion of precise time and frequency measurement <time-nuts@febo.com> >Sent: Wednesday, March 26, 2014 12:27 AM >Subject: Re: [time-nuts] RC TIC linearity correction? > > >Bob wrote: > >>I hadn't given any thought to correcting the linearity of the TIC I >>built, but my PLL plots tell me I should do it now. > >You are using a resistor to charge the integrating capacitance, so it >charges with the classic exponential curve and you get a nonlinear >time-to-voltage conversion. You need to charge the integrating >capacitance with a constant current if you want a linear >time-to-voltage function. The current source will probably need to >be connected to a supply that is higher than 5v, because it needs >some headroom. > >There may be secondary errors, as well, due to the leakage of the >tri-state buffers in their hi-Z state and/or nonlinearity in the >ADC's internal capacitors. Often you can improve things by using >sufficient external capacitance to swamp the ADC's internal >capacitance and increasing the charging current. > >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.
