Question
AD9244 what is the impedance of the analog inputs besides the 10pF mentioned
in the datasheet
Answer
The Vin+, Vin- inputs, look directly into the unbuffered sample and hold
amplifer (datasheet figure3), the inputs see a dynamic input impedance. It is
necessary to drive the AD9244 from a low impedance source with sufficient drive
capability to charge the sampling capacitors within the sampling period. A
number of strategies for driving the ADC are given in the datasheet – for
undersampling applications where the input bandwidth exceeds Fs/2 a wide band
center tapped transformer provides the optimal SFDR as well as performing
single ended to differential conversion.