ADC+Flex

[desmomito]

Hi all,
I been working on the ADC. The ADC work perfectly but the value I receive are wrong compared with the ones measured with oscilloscope. The conversion I do is (ADC_BUFFER*3.3)/4096 (I\'m using 12 bit ADC). To solve this problem I wanted to check if the dsPIC has got a 3.3V power supply. To do this I\'ve checked the voltage drop between the pins on JP6 and JP7 (JP6\'s pin = GND, JP7\'s pin = 3.3V) and I have 3.14V instead 3.3V. What\'s wrong ? Is it normal to have 3.14V instead 3.3V ?


Thank you very much,
Pasquale.

P.S. : This happens when the dsPIC is working. If I reset the dsPIC I have 3.2V instead 3.3V.

[paolo.gai]

Hi Pasquale,

- Are you usinga felx full or a flex light?
- Is the board comnnected to other sensor or actuators?

Ciao,

Paolo

[desmomito]

Hi,
yes I\'m sure that is the Flex-Full board. And I don\'t have anything connected to the board. I\'ve checked better and the voltage is 3.2V when the dsPIC is in reset state and almost 3.165 when the dsPIC is on.

Is it normal this behaviour ?

Because this really compromise the measure I do from the analog input. I should use a correct value instead of 3.3V when I do the conversion between integer and float (ADC*V_max)/4096. Otherwise I\'ll introduce an offset which is really important in term of precision.

Thank you,
Pasquale.

[paolo.gai]

Well... I think so.

If a current is flowing because the dsPIC is on, then you\'ll measure 3.3V-R*I, where R is the resistance between the 3.3V and the point where you measure (which is not 0), and I is the current drained by the dsPIC.

Bye,

PJ