If the pots work but not "perfectly" then I would guess that you are
missing the ground connection to the part. This can be subtle because
the bypass capacitor can charge up from the digital inputs and power the
part, albeit poorly.

-- Rich

Andrew Farrar wrote:
>
> Hello,
> We have rigged up 2 digital pots., analog devices AD8403.  Each
> device has a shift register, which enables daisy chaining.  Each
> device contains 4 pots which all work as expected.  Each pot. is
> addressed via the serial input.  However on inspection of Serial out
> of a device is not a delayed version of its  input, as might be
> expected from a conventional shift register.
> Output data appears spikey ie not clean switching of logic levels and
> does not go down to zero.
>
> Andy Farrar
> andrew.farrar@port.ac.uk