input filter capacitor value

Question

Hi, 
 in datasheet 
 Figure 41. Typical Application Circuit Diagrams. 
 When input is driven with differential amp ADA4945-1, there is 270pF connected between IN0+ and IN0-, but when the input is driven by ADA4896, there is no such differential capacitor, why? 
 And why the capacitor value between IN0+ to GND, and IN0- to GND is 560pF, but the differential cap between IN0+ and IN0- is 270pF, how to calculate the right value for these capacitors?

JoanOrtega · Accepted Answer

Hello howardaaa 
 The typical application circuits shown in figure 44 are just two examples to show the capability of the device to drive both, single ended and differential amplifiers. It doesn't mean that they are equivalent circuits. 
 Actually, in the examples shown, the main contributor for the differential configuration is the 2.7nF capacitor while in the single ended is the 10nF input capacitor. 
 The calculation of the components depends on your sampling frequency since you have to make sure that the signal settles correctly, the noise attenuation you want to have for you input signal. 
 I would recommend you use the following website tools to calculate the optimum values of the signal chain: 
 Signal Chain Designer | Precision Studio | Analog Devices 
 Precision ADC Driver Tool | Analog Devices 
 Regards 
 Joan)

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input filter capacitor value