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Wrong operation when using internally generated negative voltage reference for A/D converter on PIC16F18877

Mila_G

Hello,

please can you help me with the wrong operation when using internally generated negative voltage reference for A/D converter on PIC16F18877?

I would like to use ADC with references 0.416 V and Vdd (5 V).
It works perfectly when I connect 0.416 V to the Vref- pin (RA2) from an external source.

Then I generate 0.416 V on RA2 from FVR and D/A converter as follows:
  FVRCON = 0x84;
  DAC1CON0 = 0xA8;
  DAC1CON1 = 0x0D;
The DAC also works perfectly and there is about 0.416 V on the Vref- pin (RA2).

But now ADC works only if the negative reference is Gnd. If I switch this from Gnd to Vref- (just by changing ADREF from 0 to 0x10), a failure occurs. The voltage on the RA2 pin changes to 2.1 V (and of course the A/D gives bad results).

Workaround attempts:
- 0.416 V output from DAC to RB7, directly conneted to RA2 via a wire, works just as badly as above.
- After interconnecting via an op amp voltage follower instead of the wire, the A/D starts to work properly.

I assumed the PIC analog circuits were designed for internal connections from the VRef- pin, without any external workaround. Please, advise.

Many thanks.

Mila.

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  • 0

    ADC input signals are really sensitive to source impedance, and it's generally recommended to keep the maximum impedance of your analog signal source around 10 kΩ. If the source impedance is too high, it can lead to longer sampling times, which may mess up your conversion results. This could be one of the issues you're facing when using an internally generated negative voltage reference.
    To avoid problems, make sure your circuit design doesn't introduce extra impedance that could affect the ADC's performance. For more detailed information, check out the PIC16F18877 datasheet, especially the sections on ADC configuration and operation.

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  • 0

    ADC input signals are really sensitive to source impedance, and it's generally recommended to keep the maximum impedance of your analog signal source around 10 kΩ. If the source impedance is too high, it can lead to longer sampling times, which may mess up your conversion results. This could be one of the issues you're facing when using an internally generated negative voltage reference.
    To avoid problems, make sure your circuit design doesn't introduce extra impedance that could affect the ADC's performance. For more detailed information, check out the PIC16F18877 datasheet, especially the sections on ADC configuration and operation.

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