[time-nuts] RC TIC linearity correction?
bob at evoria.net
Wed Mar 26 11:41:15 EDT 2014
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.
> From: Charles Steinmetz <csteinmetz at yandex.com>
>To: Discussion of precise time and frequency measurement <time-nuts at febo.com>
>Sent: Wednesday, March 26, 2014 12:27 AM
>Subject: Re: [time-nuts] RC TIC linearity correction?
>>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
>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.
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