A/D conversion problem in 18F2520, 2523, 2550 etc.

[Acetronics2]

Hi,

While carefully reading your Datasheets ...

Did you notice the ADC input impedance had slightly changed ??? ... take care to the voltage source impedance !!! ( in your RC filters ... there's a series resistor !!! )

Your "symptoms" make me think it's a source impedance problem ...

Also consider the voltage offset of your buffers ... An AOP gives easily 10 mV offset !!! and from a batch to another, it can widely differ.

Could you show us a scheme of your input circuits ???



I Use a 2520 for my Honda Lawn tractor ( read air and oil temp - LM335 , plus oil pressure - MPX 5700 , with the inboard ADC) and readings are rock steady ...
My ref is a 4.096v from a TL431a ... trimmed with a 4.5 digits multimeter ...

The initial project was built around a 18F452 ... and I didn't notice any change.

Alain