The AD9684 has an input buffer than can be configured to improve performance at
different input frequencies. The available controls are:
- Input Termination: The input termination is programmable from 50 - 400Ù
differential. This is intended to make the filter design and implementation
easier for certain applications.
- Buffer current: The buffer current control can be used to improve the SFDR
performance of the AD9684 at various frequency bands. This is shown in figure
41 of the datasheet. As the buffer current is increased, it will draw more
power from the AVDD3 power supply. This relationship is shown in Figure 43 of
Capacitive loading: The capacitive loading at the buffer inputs of the AD9684
can be selected to either a 3pF to GND or 1.5pF to GND. For low frequencies up
to the first or second Nyquist, it is ok to use the default setting of 3pF.
This setting provides the best SFDR performance in the first and second Nyquist
zones. For frequencies > 1GHz, it is recommended that the 1.5pF capacitive
loading setting be used. This enables the driver to drive a lesser load at high