How can I make my measurements of full-wave rectified AC using an A/D match a Fluke meter on its DC range?
To follow up on @glen_geek's insightful comment, the input circuit to the Fluke on a DC range typically looks like a high value divider plus a capacitor. 







simulate this circuit &ndash; Schematic created using CircuitLab

When the diodes of the bridge rectifier are 'off', your meter actually outputs a small voltage (in the case above it would peak at 1/10 of the peak input voltage then decrease as the capacitor discharges) so the reading on the meter would be a bit higher than if you had a light load (say 100K) on the output of the bridge rectifier. 

This may be the cause of your discrepancy. The DMM won't read reliably under those conditions- a bit of diode leakage can change the reading, you really need to have a small load on the bridge rectifier output. If your ADC input has much less input capacitance you will be getting a different value (possibly lower).