> are the ADCs on the PICs just fairly crappy ?

They're OK

To get the best out of them you need quiet supply rails - Vcc,
Vdd and Vref. Depending on other PIC I/O activities you
might want to time those away from sampling. Use the greatest
resolution available, eg use 5.12V for Vref (Vcc 5.12V or a
little higher) to get 4mV / bit or use an op amp to amplify a
narrower signal range (this would also meet the impedance
spec if the sensor is hi-Z). Although some time ago there was
a thread about higher impedance signals working with longer
sampling times


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