auto tuning

[RockeyMurad]

when we are trimming a resistor this permits us to use low tolerance capacitors C+ΔC what exactly is Δc and why we don't have to know it

 

[sidun.av]

I don't really understand your question. We can tune both resistors and capacitors, if neccessary. In the most of the capacitor-based circuits (for example, capacitive SAR ADC arrays, SC-amplifiers and filters) only the relation between capacitor values matters, i.e. C1/C0 etc. We can keep that relation by using unit-sized caps and matching them in layout.
For example:
C₁ = 4*C₀
So,
C₁/C₀ = 4*C₀/C₀ = 4
Let's add some parasitics:
(C₁ + ΔC₁)/(C₀ + ΔC₀) = 4*(C₀ + ΔC₀)/(C₀ + ΔC₀)
(4*C₀ + 4*C₀) /(C₀ + ΔC₀) = 4

Hopefully, that helps.