How do you make ADC input significantly lower than Vss?

There is no simple solution, particularly where cost is an issue.

A few details to consider about the nature of the applied voltage might be:

  • How high/low is its impedance?
  • Is it DC?
  • Is it a transient? Look at the profile details.
  • Is it slowly varying? Look at the profile details, if the transient is fast enough, using a larger RC may allow it to be filtered.

Protecting the PIC® MCU itself requires something between the PIC MCU and the negative voltage. Therefore, attenuation of the signal has an impact. Finally, you must decide what is tolerable and how many bits of resolution are truly required.

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