[time-nuts] RC TIC linearity correction?

Bob Stewart 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?
>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,

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