I am using DSO90804A for some delta time measurements in the order of 50ps. The scope datasheet lists the measurement error limit in pg-26.

Is the error limit applicable only when sin(x)/x interpolation is enabled? Or even with no interpolation enabled?

Have you done the calculation for the 'Delta Time Measurement Accuracy'? Unless you have really fast edges, I doubt that the scope you have will work well enough. Note that you also have to make sure that you have removed any delay effects from your fixture an cabling. 50 psec is about 1/3 inch in a cable so there is a possibility that the inaccuracies in your measurement system will overwhelm the measurement you are trying to make.

All noise and Slew Rate measurements assume the Sample Rate is at Max, and Sinx/x interpolation is enabled, so that does indirectly affect the error limit of your measurement.

Al