Removing "Offset" From ReadAD10

[William Roth]

Removing Low Range offset from READAD10 Result

Background:
With a PIC18F25K22 the 10-Bit ADC Result has a positive offset of 2, even when the ANx PIN is tied directly to the PIC Ground pin. With a quality 10K pot the result swings from 2 to 1023. It should swing from 0 to 1023. I am not sure if this is a problem with GCB "ReadAD10" or something else.

How to do it:
In any case, to remove this "offset" I used the following formula borrowed from the Arduino "MAP" command.

return (x - inmin) * (outmax - outmin) / (inmax - inmin) + outmin

With the values plugged in it becomes:

adc_val = (adc_val - 2) * (1023 - 0) / (1023 - 2) + 0

Simplified, it becomes:

adc_val = (adc_val -2) * 1023 / 1021

Example Code (Tested to Work):

#chip 18F25K22, 16
#config MCLRE = INTMCLR
#Config XINST = OFF
#config OSC = INTIO67

;----- Setup LCD Parameters
#define LCD_IO 4
#define LCD_NO_RW
#define LCD_Speed fast

; ----- Define Hardware settings
#define LCD_RS PORTC.2
#define LCD_Enable PORTC.3
#define LCD_DB4 PORTC.4
#define LCD_DB5 PORTC.5
#define LCD_DB6 PORTC.6
#define LCD_DB7 PORTC.7

; ----  Pins and Variables
DIR PORTA.5 IN
DIM adc_val as LONG

; ----- Main body of program starts here.
CLS
do
    Locate  0,4
    adc_val = (ReadAD10(AN4))                 'Raw ADC reading
    adc_val = (adc_val - 2) * 1023 / 1021      'Remove Offset
    Print adc_val
    wait 1 s
    Locate 0, 4
    Print "          "
loop

END

[polyconnect]

Very useful stuff. Thanks !