El ohmímetro no funciona como se esperaba

Estoy tratando de construir un ohmímetro simple creando un divisor de voltaje y luego usando un pin analógico en un PIC18F2550 para leer el voltaje de salida y determinar el valor óhmico de una de las resistencias. Estoy usando un oscilador de 20 Mhz para el reloj del PIC, y todo el circuito funciona con un regulador LM7805. Utilizo una resistencia permanente de 10K ohmios como resistencia 2 en mi divisor y resuelvo el valor de la primera resistencia. Estoy leyendo el valor del PIN AN0. El problema es que el valor leído es muy impreciso y no estoy seguro de cuál es el problema. ¿Qué podría hacer para obtener lecturas más precisas?

Aquí está mi código para el compilador XC8

#include <stdio.h>
#include <stdlib.h>
#include <xc.h>
#include <string.h>

#pragma config FOSC = HS        // Oscillator Selection bits (HS oscillator (HS))
#pragma config FCMEN = OFF      // Fail-Safe Clock Monitor Enable bit (Fail-Safe Clock Monitor disabled)
#pragma config IESO = OFF       // Internal/External Oscillator Switchover bit (Oscillator Switchover mode disabled)
#pragma config PWRT = OFF       // Power-up Timer Enable bit (PWRT disabled)
#pragma config BOR = OFF        // Brown-out Reset Enable bits (Brown-out Reset disabled in hardware and software)
#pragma config BORV = 0         // Brown-out Reset Voltage bits (Maximum setting)
#pragma config VREGEN = OFF     // USB Voltage Regulator Enable bit (USB voltage regulator disabled)
#pragma config WDT = OFF        // Watchdog Timer Enable bit (WDT disabled (control is placed on the SWDTEN bit))
#pragma config CCP2MX = OFF      // CCP2 MUX bit (CCP2 input/output is multiplexed with RC1)
#pragma config PBADEN = OFF      // PORTB A/D Enable bit (PORTB<4:0> pins are configured as analog input channels on Reset)
#pragma config LPT1OSC = OFF    // Low-Power Timer 1 Oscillator Enable bit (Timer1 configured for higher power operation)
#pragma config MCLRE = OFF      // MCLR Pin Enable bit (RE3 input pin enabled; MCLR pin disabled)
#pragma config STVREN = OFF     // Stack Full/Underflow Reset Enable bit (Stack full/underflow will not cause Reset)
#pragma config LVP = OFF        // Single-Supply ICSP Enable bit (Single-Supply ICSP disabled)
#pragma config XINST = OFF      // Extended Instruction Set Enable bit (Instruction set extension and Indexed Addressing mode disabled (Legacy mode))
#pragma config CP0 = OFF        // Code Protection bit (Block 0 (000800-001FFFh) is not code-protected)
#pragma config CP1 = OFF        // Code Protection bit (Block 1 (002000-003FFFh) is not code-protected)
#pragma config CP2 = OFF        // Code Protection bit (Block 2 (004000-005FFFh) is not code-protected)
#pragma config CP3 = OFF        // Code Protection bit (Block 3 (006000-007FFFh) is not code-protected)
#pragma config CPB = OFF        // Boot Block Code Protection bit (Boot block (000000-0007FFh) is not code-protected)
#pragma config CPD = OFF        // Data EEPROM Code Protection bit (Data EEPROM is not code-protected)
#pragma config WRT0 = OFF       // Write Protection bit (Block 0 (000800-001FFFh) is not write-protected)
#pragma config WRT1 = OFF       // Write Protection bit (Block 1 (002000-003FFFh) is not write-protected)
#pragma config WRT2 = OFF       // Write Protection bit (Block 2 (004000-005FFFh) is not write-protected)
#pragma config WRT3 = OFF       // Write Protection bit (Block 3 (006000-007FFFh) is not write-protected)
#pragma config WRTC = OFF       // Configuration Register Write Protection bit (Configuration registers (300000-3000FFh) are not write-protected)
#pragma config WRTB = OFF       // Boot Block Write Protection bit (Boot block (000000-0007FFh) is not write-protected)
#pragma config WRTD = OFF       // Data EEPROM Write Protection bit (Data EEPROM is not write-protected)
#pragma config EBTR0 = OFF      // Table Read Protection bit (Block 0 (000800-001FFFh) is not protected from table reads executed in other blocks)
#pragma config EBTR1 = OFF      // Table Read Protection bit (Block 1 (002000-003FFFh) is not protected from table reads executed in other blocks)
#pragma config EBTR2 = OFF      // Table Read Protection bit (Block 2 (004000-005FFFh) is not protected from table reads executed in other blocks)
#pragma config EBTR3 = OFF      // Table Read Protection bit (Block 3 (006000-007FFFh) is not protected from table reads executed in other blocks)
#pragma config EBTRB = OFF      // Boot Block Table Read Protection bit (Boot block (000000-0007FFh) is not protected from table reads executed in other blocks)

#define _XTAL_FREQ 20000000

int main()
{
    ADCON0 = 0b00000001;
    ADCON1 = 0b00001110;
    ADCON2 = 0b10001010;
    TRISA0 = 1;

    while(1)
    {   
        GO_DONE = 1;
        while(GO_DONE);
        unsigned int adc = ((ADRESH<<2) | ADRESL);
        const float maxAdcBits = 1023.0f;
        const float vin = 5.0f;
        const float resistance2 = 10000.0f;
        float voltsPerBit = (vin / maxAdcBits);
        float vout = adc * voltsPerBit;
        float ohms = ((resistance2 * vin) - (resistance2 * vout)) / vout;
    }
}