Regarding design of ADC

[NARENDRA1234]

ii am designing 12 bit ADC.its MCP3204/3208.i am attaching its datasheet.we want to convert valtage range from .2v to .8 volts into digital signal fwhich will bi given to microcontroller.so how to do it?how its calculations are done?

 

[manojdharap]

Hi Narendra,
Assuming your PS voltage is 5V

1) Use low ripple LDO to convert Power supply voltage to somewhere between 2.5V to 3.3V from 5V. and connect O/P of LDO to Vref pin
2) Now use signal processing/filters to make I/P signal noise free and also amplify the max I/P to 90-95% of Vref. (eg. 0.8V to 3.2V for your application and then connect it to any channel I/P
3) To configure channels as well as serial communication go through 5.0 section of data-sheet.

I hope this is useful

 

[NARENDRA1234]

is it necessary to uce LDO? how to calculate op of ADC?

 

[manojdharap]

Hi Narendra,
It is not necessary, normally it is used to reduce noise in Vref by additional regulator so that ADC O/p will be noise free.
To calculate ADC O/P,
For 12 bit ADC, max count is 4096.
Now 4096 is equal to Vref,
so in your case, for given I/p you can calculate O/p count = (Vinput*4096)/Vref. eg. For For 3.3V Vref & 1V I/P, O/p will be 04D9 Hex

I hope this is clear...

 

[randell_xtian]

Hi.

first..you can used directly 5V as a reference to the ADC. the formula is ADC = (4096 * Vin)/Vref... where the 4096 is equal to 2^12. for example ADC = (4096 * 0.8V)/5V... the ADC value should be... 655 decimal or 28F. However, the range of ADC is small if you used a 5V as a reference voltage... so that manojdharap suggest to lower your reference voltage to 2.5 to 3.3V. thus ADC = (4096 * 0.8V)/3.3V... equal to 993 in decimal or 3E0.

 

[NARENDRA1234]

ok..thnks man i got an idea..now suppose i have an input of 0.8volts and i want to it to be converted to 400 numbet into decimal and displayed it on LCD.i have PIC with inbuilt ADC and LCD display.how will it can be done?

---------- Post added at 12:40 ---------- Previous post was at 12:38 ----------

ok..thnks man i got an idea..now suppose i have an input of 0.8volts and i want to it to be converted to 400 numbet into decimal and displayed it on LCD.i have PIC with inbuilt ADC and LCD display.how will it can be done?

 

[randell_xtian]

what model of micro (PIC) would you used? its 8bits, 16bits or 32bits?

i'm using PIC 18 series and HI-TECH C code software to program the micro...

#include <pic18.h>
#include <hitech.h>
#include <adc.h>

int ADC_VALUE; /* The value from the analog - digital convertor */
void main(void)
{
TRISA=0x3f; /* PORTA as Anlog input */
TRISD=0x00; /* PORTD as output */
while(1)
{
ADCON0=0b1000011; /*the osc F bits 7-6 | the channel bits 5-3 | start bit2 | none bit 1| A2D on when "1" bit 0| */
ADCON1= 0x00; /* 10 bit / 8 bit convertor when "0x10" 10 bit - "0x00" 8 bit */

ADIE = 0; /* Masking the interrupt */
ADIF = 0; /* Resetting the ADC interupt bit */
ADRESL = 0; /* Resetting the ADRES value register */
ADRESH = 0;
GODONE=1; /* Staring the ADC process */
while(GODONE); /* Wait for conversion complete */
ADC_VALUE = (ADRESH); /* Getting HSB of CCP1, in 8 bit mode only
ADRESH in 10 bit mode need the ADRESL to
perform the first 8 bits, THE ADRESH perform the 8
last bits*/

PORTD = ADC_VALUE; /* perform in PORTD the Value */
}
}


Moreover, it is the direct convertion of your analog sample to readable digital sample (ADC convertion only). if you would like to display your sample in a LCD... just call the variable ADC_VALUE... example:

lcd_puts("%d",ADC_VALUE);

remember on LCD...you must first initialize the LCD before it work!