[time-nuts] RC TIC linearity correction?
csteinmetz at yandex.com
Wed Mar 26 01:27:56 EDT 2014
>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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