Question about split cap array ( for SAR ADC)

[AOQ]

Dear all :

In my technology, about 50fF unit cap can achieve 10 bit resolution for CDAC type
SAR ADC. Now if I use split cap to reduce total capacitance, the result is bad.

Does anyone know what the effect for using split cap array ?
How large unit cap shoud I use ?

thx~~

 

[mists]

maybe resistor mix capacitor is better than cap split.