In ADRF6510 datasheet Figure 56, the anti-aliasing filter choose 500ohm load
resistor to define the input impedance of the ADC.
Why it choose 500ohm?
Is it easier to design matching circuit?
Is 200ohm ok ??
Figure 56 is a design example. The 500 ohm impedance defines the impedance seen by the ADRF6510. Also, the anti-aliasing filter needed to be designed to a impedance of some sort (in the design example) and 500 Ohms is a good trade off of providing a defined impedance while not reducing the absolute gain too much.
The ADRF6510 probably won't be able to drive 200 Ohms as well; you'll lose some gain if driving 200 Ohms. I would recommend designing to an impedance greater or equal to 500 Ohms.
Thank you Sir.
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