ADS1298 Input Model

I am working on an application of the ADS1298 using +/- 2.5V analog supplies and 3.3V digital supply.  I need to develop a small signal spice model for use at low frequencies (10-100 Hz) to evaluate front end filter performance.  I see the typical input capacitance is listed as 20 pF. 

Questions:

1. Do you have a spice model of the inputs that you would recommend?
2. Should I model this capacitance as 20 pF to Avss for each input or 20 pF between the pos and neg inputs?
3. What is a practical estimate for how much this 20 pF can vary with different production lots?

Thanks, Eric