Philip Pemberton wrote:
> Both Vcc's are bypassed with 100nF ceramics
> within millimetres of the chip, grounding straight to the topside
> ground plane.

This isn't the cause of your problem, but that's wrong.  Bypass caps should
be connected between the power pins, with a single connection for the whole
chip to the ground plane.  If you've got separate connections to the ground
plane, especially when those connections are bypass caps, you are running
high frequency currents from the chip accross the ground plane.  That's bad
since the signals are much more likely to radiate, and other ground plane
currents now cause offsets to the voltage the chip sees.


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