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ADC Pin of PIC 18F4685 Loading Circuit Unacceptably

macaulay.flower

Feb 25, 2012
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Feb 25, 2012
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Hello there!

For a school project, I am creating a simple data acquisition system which uses a frequency to voltage converter to sample a frequency and then display it on an LCD screen on my PIC 18F4685 board.

I have created the frequency to voltage converter in such a way as that it outputs a voltage from 0-5V for an input of 0-1000Hz.

When I attach this frequency to voltage converter to the ADC pin (RA0) of the board, the voltage drops from 5V to 4.7 volts (as seen on a multimeter and the LCD screen). To fix this, I added a buffer circuit which consists of a voltage follower (with an op amp). However, this buffer actually makes the problem worse - that is, it drops from 5V to about 0.6 V.

How can I create this circuit so that the PIC board does not load it so much?

Thanks for any ideas.
 

Harald Kapp

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Nov 17, 2011
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This shouldn't happen.
1) What is the output voltage of the voltage follower without the PIC connected?
2) Did you make sure to use a rail-to-rail OpAmp for your analog circuit? If you power a non rail-to-rail OpAmp from 5V, the output will not be able to reach +5 V or 0V (see OpAmp datasheet for exact values).
3) Did you ensure that the port pin of the PIC is configured as analog input to the ADC?

Regards,
Harald
 
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