Ohmmeter funktioniert nicht wie erwartet

Ich versuche, ein einfaches Ohmmeter zu bauen, indem ich einen Spannungsteiler erstelle und dann einen analogen Pin an einem PIC18F2550 verwende, um die Ausgangsspannung zu lesen und den Ohmwert eines der Widerstände zu bestimmen. Ich verwende einen 20-MHz-Oszillator für die Uhr des PIC, und die gesamte Schaltung läuft von einem LM7805-Regler ab. Ich verwende einen permanenten 10K-Ohm-Widerstand als Widerstand 2 in meinem Teiler und löse nach dem Wert des ersten Widerstands auf. Ich lese den Wert von PIN AN0 ab. Das Problem ist, dass der gelesene Wert sehr ungenau ist und ich nicht sicher bin, was das Problem ist. Was kann ich tun, um genauere Messwerte zu erhalten?

Hier ist mein Code für den XC8-Compiler

#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;
    }
}