1. how to calculate the absolute value of capacitance according to the raw data? Is there a equation to do this? Since the resolution is 0.1pF, i assume the detection range is 0 to 25.5pF? Is that correct?
2. For the sensor we are going to use, 100kHz will be a proper frequency for the stimulation signal. So, how to set the stimulation signal frequency or can we use external signal source to drive the sensor? What is the frequency range it support?
The relation will depend on which configuration you are using. The general equation is as follows Rawcount=((2^N − 1)*VREF* FSW* Cp) / IMOD You may find the details of this equation in the design guide in the following link
I believe you are referring to FSW. If so then this can be configured by setting dividers internally. If you using XTAL for IMO then this signal will be derived from that else you may not be able to use external signal for this.