Current sensor output modification. PIC or opamp or something else?

[mpgmike]

Back to the original challenge, I'd do it in software:

IF ADC > 127 THEN  ;Assuming 8-bit ADC
  ADC_VAL = (ADC * 4) / 5  ;Yields 80% of value
ENDIF

The above snippet will deliver <128 results as soon as ADC > 128. I don't think that's what you want. So we have to add a filter:

IF ADC > 127 THEN  ;Assuming 8-bit ADC
  B0 = ADC - 127  ;Use a temporary variable to determine how much above 128 our result is
  B1 = (B0 * 4) / 5  ;Get 80% of only the amount > 128
  ADC_VAL = B1 + 128  ;Yields mid-point + 80% of overage
ENDIF