Lectura de voltaje negativo con PIC16F887 PIC C Compiler

Programación lenguaje C #include <16F887.h> #device ADC=16 #FUSES NOWDT //No Watch Dog Timer #FUSES NOBROWNOUT //No brownout reset #FUSES NOLVP //No low voltage prgming, B3(PIC16) or B5(PIC18) used for I/O #use delay(internal=4000000) #include void main() { setup_adc_ports(sAN0); setup_adc(ADC_CLOCK_DIV_2); int32 valordigital; float valoranalogico; lcd_init(); // set_adc_channel(0); lcd_gotoxy(1,1); printf(lcd_putc,"Nivel de voltaje: "; while(TRUE) { valordigital=read_adc(); valoranalogico=5.00*valordigital/65536.0; lcd_gotoxy(1,2); printf(lcd_putc,"- %f V ",valoranalogico); } } Diagrama proteus 8