I'm using the ADA4937 as a buffer/driver for the AD9649BCPZ-40 ADC.

My system is required to have 1k input impedance. The current line-up is: input -> anti-alias LPF -> ADA4937 -> differential RLC LPF -> AD9649BCPZ-40.

Would it be better to design the ADA4937 with 50 ohm input impedance and design the anti-alias LPF to transform from 1k to 50 ohm, or should I just design everything at 1k? Required bandwidth is less than 10 MHz but I need it to be low noise.

Thanks,

Dave

Hello Dave,

It would be somewhat difficult to make a 50 ohm differential input impedance with a diff-amp at low gains (a buffer application) because the required feedback and gain resistor values would have to be less than the minimum requirement. For 50 ohms input impedance Rg would have to be 25 ohms and Rf=25 ohms for gain=1 or Rf=50 ohms for gain=2. The recommended value of Rf is 200 ohms for the ADA4937 from the datasheet. You need to assume this is the minimum value. You can increase it but need to avoid lowering it. So, the recommendation is to design everything for 1k.

Zoltan