Hi,
This question is probably best answered by the high speed amplifier team, but seeing as it is an ADC you're interfacing to I'll give it a shot. You may want to post follow-up questions to the high speed amplifier forum.
We try to isolate the capacitive load to keep the amplifier stable. In the case shown in the datasheet, it was found that a 125-Ω series resistor on each leg was sufficient to isolate the 10-pF load (2.2 external to ADC, 8 pF internal to ADC). In your circuit you've added some additional filtering, so it is a good idea to simulate the circuit to verify stability and correct frequency response. You can do this by using TINA-TI. There is a great blog series that should help you get started - see the link below.
One important point is that the input load of the ADC needs to be considered. Namely, there will be a capacitive load at the input of the ADC that will effect your frequency response. Please see the following app note for more information.
The way you have the circuit setup now is probably not the correct way to do it. You already have 50 Ω resistors per leg to isolate the amplifier output from the filter, so you don't need additional isolation at the ADC - at least not 125 Ω worth. The 125-Ω resistor and 2.2pF capacitor (plus ADC internal capacitance) will form a low pass filter in addition to the filter you already have in place. We will often put some small resistors (5-10 pF) at the ADC input to isolate the kick back noise from the track and hold, but this often requires iteratively changing them to find the best performance. I would remove the 125-Ω resistor and 2.2 pF capacitor and then add in a small 5-Ω resistor in series with the ADC inputs. You should then simulate the circuit to verify stability.
Regards,
Matt Guibord
