Getting some unexpected results (and very much un-needed project
delays) trying to use the 18F1320's A/D and Vref+

The initial problem was that the A/D result was coming out a little low,
but investigating that has led to perhaps another

Vcc = 5.40V
Vref+ = 5.12V, applied to AN3/Vref+ pin
256R (4-20mA load) from AN0 to Vss

8MHz INTOSC (verified by timing tests)
TRISA.3 and .0 as inputs
ADCON1.3 and .0 as analogue
01 in VCFG<1:0>, for Vref+ and Vss
Channel0 permanently selected

Firstly, results are low. For example, an input voltage of 1.385V,
as measured at AN0 pin, should return 0x114. What is returned is
0x103, and similarly differences with other input voltages

Then next thing I notice is that Vref+ seems to be being ignored. If
it's removed from AN3 and a pull-down used, the A/D still functions
as above. If VCFG is set for Vcc and Vss, the A/D output is still
low

I've looked through the datasheet and all the errata I can find, and tried
several 18F1320 (date code 0745) straight out of the tube, but I'm still
none the wiser as to how to understand and fix this

Any suggestions ?

TIA

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