[time-nuts] RC TIC linearity correction?

[Charles Steinmetz]

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