A good setting of a reference voltage can be considered a must where precision is an important matter. 90% of projects are based assuming the VDD (microcontroller 5V power supply) as voltage reference but when the voltages to be compared are very small or the precision is a critical factor, very probably, it is better to reduce the value of this reference.

Synthesizing, we can reduce the different possibilities to obtain a reference supply voltage to the following: 

• using the 5V regulated power supply voltage, normally obtained by the use of a positive voltage regulator (7805 precision 4%, according to datasheets) 
• using dedicated components as micropower voltage reference diodes, as the LM385 for instance (a 1.235V or 2.5V zener diode,  1% precision).

The first solution is quite common where absolute precision is not a critical factor, the second regards components not easy to find and sometimes a little difficult to manage because of the calculation of the conversion procedure (normally an 8 bit microcontroller divides the reference voltage in 1024 steps because of the 10bit adc).

A really economical and easy solution can be found using a normal multiturns trimmer. In this example we'll show the value in degrees of the temperature using an LM35, a precision centigrade temperature sensor (LM34 for farheneit temperatures) according to the following schematic using a 16F877 (reading the datasheet of the microcontroller, we'll discover that the minimum level of Vref+ is around 2.5V). 

As we know, according to the datasheet of the component, the LM35 provides a voltage of 10mV every centigrade of temperature; this means only 1V at 100°c therefore the value of the vdd voltage, as reference, can be considered too high for our needs.

	ADCON0 will be set as: 10 (fosc/32) 010 (RA2/AN2 as analog input) xxx ADCON0 = 0b01010000 =0x90
	ADCON1 will be set as: 1 right justified xxx 0001 Vref+ = RA3 all analogical ADCON1 = 0b10000001 = 0x81 At this point we can start to write our program (we are using C but the code, with small modifications, can be used with any other language)

Ff the Vref+ has been set to 2.5V mV the conversion is: ADC_read_value = 2500/1024 = 2,441 (mV/step).

The LM35 provides 10mv/degree therefore the conversion will be very simply: ADC_read value * (2,441*1000 to avoid float point math) /1000*10(mV/degree).

Let's start. To check the validity of the results it is suggested to replace, initially, the LM35 temperature sensor with a trimmer connected to the 5V power supply simulating a temperature value.

/-------------------------------- LCD connections, could be different

sbit LCD_RS at RB4_bit; 
sbit LCD_EN at RB5_bit; 
sbit LCD_D4 at RB3_bit; 
sbit LCD_D5 at RB2_bit; 
sbit LCD_D6 at RB1_bit; 
sbit LCD_D7 at RB0_bit; 
sbit LCD_RS_Direction at TRISB4_bit; 
sbit LCD_EN_Direction at TRISB5_bit; 
sbit LCD_D4_Direction at TRISB3_bit; 
sbit LCD_D5_Direction at TRISB2_bit; 
sbit LCD_D6_Direction at TRISB1_bit; 
sbit LCD_D7_Direction at TRISB0_bit;

unsigned int adc_value; 
long temp; 
char value [15]; 
char *degs = "00.00"; 
void main() 
{ 
ADCON0 = 0b01010000;                               // Fosc/32, RA2 input 
ADCON1 = 0b10000001;                               // Ra3 = Vref, all analog = 2.5V 
TRISB = 0x00;                                             // PORTB All Outputs 
TRISA = 0xFF;                                             // PORTA All Inputs
Lcd_Init(); 
Lcd_Cmd(_LCD_CLEAR); 
Lcd_Cmd(_LCD_CURSOR_OFF); 
do     
    {
     adc_value = ADC_Read(2);                       // channel2 (RA2)      
     temp = ((long)adc_value*2441)/100;           // 2.5V/1023 =  (adc* 2,443 * 1000)/100 (10mV *°C)      
     LongToStr(temp,value);                            // display value 'brutely'
     lcd_out(1,1,"Temp.:");      
     lcd_out(2,5,value);           
     degs[0] = temp/1000 + 48;                        // display formatted value in degrees 
     degs[1] = (temp/100)%10 + 48;      
     degs[3] = (temp/10)%10 + 48;      
     degs[4] = (temp)%10 + 48;      
     Lcd_Out (3,12,degs);
     delay_ms(300);      
     } while (1);

}