Magnus said, It would be interesting to test the linearity of the TIC separately for
instance. I have done some testing of the TIC. It works quite well for the Arduino Uno with its 1 V full scale ADC setting, but the exponential shape of the RC charging from the 5 V is quite evident if you use a processor like the Micro's 32u4 with a 2.56 V sensitivity. This circuit is also sensitive to any noise on the 5 V supply. The linearity is not too important for this application as long as it does not affect the loop gain so much that the loop becomes unstable. However I have modified the circuit to use a simple 2 ma current source as shown in the attached schematic, with significantly improved linearity. H/T to Horowitz and Hill's The Art of Electronics for the idea of using an LED as the voltage reference for the current source. The diode-connected 2N3904 has less leakage at a small forward voltage than any small-signal diodes I tested, so the output voltage with a very narrow input pulse is close to zero. I could run some tests on my version by setting the oscillator slightly off-frequency and capturing the resulting sloped TIC output, and would be happy to share the results if you are interested.
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