I stumbled across this line in this [datasheet](http://www.analog.com/static/imported-files/data_sheets/AD5290.pdf) (page 10):

> 
> Serial Data Output Pin. Internal N-Ch FET with open-drain output that
>  requires external pull-up resistor. It shifts out the previous eight
>  SDI bits that allow daisy-chain operation of multiple packages.
> 
> 
> 

Could somebody briefly explain what this means? Specifically, if I'm not planning on using the daisy-chain functionality, can I leave this pin unconnected or do I need to connect it to an external pull-up resistor regardless?