A/D conversion problem with pic16F88

[Acetronics2]

Hi,Tapio

Could also come from an unwanted 10 to 8 Bits truncature ( indexed EEPROM transfer bug i.e. )... I do not see why, but it is possible ... so, to be checked !!! .

Alain.

[paul borgmeier]

Tapio,

Another thing to try ....

Remove the thermocouple input from RA2 and replace it with a known voltage input (from a voltage divider or pot). With this, you should be able to determine if the problem is from the software side or the hardware side. Just a thought ...

[Tapio Linkosalo]

Alain,

truncation is a possibility that came also to my mind. I tried adding "define adcbits 10" to the software yesterday, at that time it started to work, but it could be also that re-soldering one lead of the thermocouple at the same time was the real reason. I tried to remove the define, even erase the chip and reprogram in case that the define did a change in the configuration that was not reversable by removing it, but could not return to the erraneous state of affairs.

I wonder if the reason could be in the input. I checked the output from the op. amp. with oscilloscope, and it seems to be stable within around 5%, nothing like 4-fold variation in the input values.

[Acetronics2]

Hi, Tapio

Glad to hear it works ...

As the Define Adcbits deal with adcin ... I'd bet the thermocouple was the trustable reason ...

Alain

[Tapio Linkosalo]

Actually, it did not work, but now I think it does.

The problem was not in the A/D configuration, but the op-amp. The datasheets tell to add a resistor for bias current return path which I had not added (and also had to tinkle around as ref ground and opamp power ground must be different). So the erronous readings were due to thermocouple drifting towards V+ref end of the input.