RIGOL DM3058 - Manual (Page 120)

RIGOL
User’s Guide for DM3058/DM3058E 5-2
AC ripple present on DC power supplies. There are situations, however, where you
might want to know the AC+DC true RMS value. You can determine this value by
combining results from DC and AC measurements as shown below. You should
perform the DC measurement using 5.5 digit mode for best AC rejection.
2 2
) (
DC AC RMS
DC AC
+ =
+