The MCP3424 ADC suggested in another thread is giving very accurate results. It’s in total agreement with my Fluke 77 which is only good to three decimal places.
In a bake-off between the MCP3424 and the Imp’s ADC, the Imp with oversampling was better than I expected. Vdd and Vdda were slightly over 5v so the Imp’s reading were likewise slightly high.
Bottom line; a separate voltage reference to Vdda and oversampling and the Imp’s ADC is quite respectable. Of course it’s slow with oversampling, not differential and takes one pin per input but it could save on an external component.