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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.

  • 0

    I didn't read it all, and I have not used a PIC for many years, but looking here:

    image

    image

    image

    image

    If I've understood your requirement right (I might not have - I'm not used to these registers and other detail you mention) it's not a surprise if wiring the DAC to anything requires a buffer. The DAC appears to be composed of a ladder of resistors with no buffer from what I can tell, so yes, you'd need to add your own buffer (e.g. op-amp) when realistically connecting that externally to anything.

    If you mention your actual end requirement (including what you're actually trying to connect the ADC to), and why the external buffer is not usable, then perhaps a different or better solution could be provided.

  • 0 in reply to shabaz

    I have read the PIC specs carefully: "A buffer must be used on the DAC reference output for external connections to the DAC1OUT1/2 pins.", see above. But I would like to use internal connection! No user external circuitry conneted to the RA2 pin, no wiring, but only internal connection from RA2 to the ADC ref. input inside the MCU.

    Many thanks. Mila.

  • 0 in reply to Mila_G

    You explicitly mention in your workaround attempts:

    Workaround attempts:
    - 0.416 V output from DAC to RB7, directly conneted to RA2 via a wire, works just as badly as above.

    I'm merely explaining the pointlessness of doing that, since the datasheet makes it clear that would not be possible.

    You have a solution, which is a buffer.

    If you are unwilling to accept a workaround, then I'm no expert here but it's clear from the datasheet that what you're suggesting is not possible.

    Check the diagram below. The negative reference input to the ADC comes from either a pin, or from ground. You can see it in red.

    The DAC output (described as a DAC reference output in the datasheet) in usable internally , as an input to the channel (shown in blue below), not as a input to the ADC negative reference.

    image

    If you provided more information about your actual use-case (what you're measuring for instance) then maybe there are other solutions, but trying to make an internal connection that doesn't exist won't work.

  • 0

    You may consider doing the operation digitally. Your increasing the sensitivity 8% or 0.1 bits changing the FSR from 5V to 4.6V. You could subtract 416 mV in software. Sounds like a neat project, best of luck.

  • 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.