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ad9707 gain control with external voltage reference


I will use ad9707 output arbitrary waveform, need to digitally control output amplitude. A coarse controlling is OK, like 100~256 taps. Don't want to use VGA like AD8338.

In datasheet, there is a comment:  " Also, a variable external voltage reference can be used to implement a method for gain control of the DAC output. The external reference is applied to the REFIO pin." I want to use this method.

My question is how to implement this variable external voltage reference? What I know is LDO+digital potentiometer. Any other suggestion? Thanks!