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Front-End Amplifier and RC Filter Design for a  SAR ADC

Hi, I'm using an Instrumentation Amplifier AD8226 followed by and RC Filter to drive an unbuffered SAR ADC. The signal to be sampled is quite slow, 1KHz with a 3Vp max. I read this Analog Design's article and i got very confused if i can use the circuit i design to perform the sampling and i really need some help.

The Acquisition time of the ADC is 2uS and its maximum source impedance is 1K ohms.  Doing the math stated in the article  i get  the time constant  of 356nS.  I have two questions, is this the minimum or maximum time constant  i  need to respect and can i use a filter with this time constant between the AD8226  and the SAR Adc.

Thank you. 

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