FDC2112: How to implement the external cap?

Zhizhao,

Thank you for your inquiry and your interest in TI products.

Just so you know, we are suggesting that users consider using the FDC1004 as an alternative to the FDC2x devices. 
We have found the FDC2x devices can be sensitive to EMI.

The FDC1004 has internal active shield drivers that can help reduce the impact of EMI on the end system.

One of the more popular end applications for the FDC1004 is liquid level sensing, where the sensors are placed against the outside of the plastic tank holding the liquid to be measured. So at least for this application, the sensors work fine with a plastic surface.
An example of capacitive sensors encapsulated in a flex circuit is shown in the app note Capacitive-Based Liquid Level Sensing Sensor Reference Design.

You can see a comprehensive list of app notes, reference designs for the FDC1004 at our FDC1004 Capacitive Sensing FAQ page.

Regards,
John