8-bit MicrocontrollersのトピックRe: how to get analog-to-digital conversion accuracy??.

how to get analog-to-digital conversion accuracy??.

quimey — Thu, 15 Jan 2009 02:23:54 GMT

hello I have a problem in mi ad input : to to to much noise.

I have a potentiometer of 10k and the noise is 100 to 200 mvolt.

Some know the AN1056?

You me can send it?

Can someone help me?
i´d like to do a Algorithm for reduce the noise for software,do you know someone?

I am using C language

tank you sorri for my inglish.

Re: how to get analog-to-digital conversion accuracy??.

Lundin — Thu, 15 Jan 2009 18:06:50 GMT

Are you feeding the ADC with a separate reference voltage or with the regular supply voltage?

Re: how to get analog-to-digital conversion accuracy??.

denny_george — Thu, 15 Jan 2009 20:03:59 GMT

how to get analog-to-digital conversion accuracy??.

Step1: Read Bandgap Buffer value ( Bandgap Buffer Enable= TRUE ;-) ok)

Step 2: Bandgap Buffer value (in hex) corresponds to voltage 1.2 V (See Bandgap Voltage Reference in DC Characteristics (appendix part of datasheet.)

If ADC Resolution is n bit ==> ADC( Bandgap) value in REG= ( 1.2 / 5 ) * 2^n , then the analog-to-digital conversion accuracy is quite good.

You have not mentioned the MCU family name and ADC resolution.

Message Edited by Geo*** on 2009-01-15 12:06 PM

Re: how to get analog-to-digital conversion accuracy??.

celsoken — Thu, 15 Jan 2009 21:27:21 GMT

Dear Quimey,

I don't have enough data to help you but you probably could implement a simple software low-pass filter by averaging the results.

You can implement it by taking n samples, adding n/2 to the result and after dividing the result by n. If both n and the sampling interval is adequate, the results can be ok.

Cheers,

Celso

Re: how to get analog-to-digital conversion accuracy??.

denny_george — Thu, 15 Jan 2009 21:43:45 GMT

Dear Quimey,

How come you came to know that there is a noise of 100-200mV ? CRO ???

Read Channel BandGap ADC value .It is +/- 1, 2 counts from the eqn I said before.

Anything extra ADC value on the Channel BandGap could be considered as white noise.

Similarly you can Read Channel Vrefh= max value and Channel Vrefl= 0x000. I would suggest the Bandgap thing or Channel Vrefl= 0x000.

Message Edited by Geo*** on 2009-01-15 01:51 PM

Re: how to get analog-to-digital conversion accuracy??.

quimey — Fri, 16 Jan 2009 08:38:06 GMT

with the regular supply voltage way?

Re: how to get analog-to-digital conversion accuracy??.

quimey — Fri, 16 Jan 2009 08:39:42 GMT

the mcu family y mr and the resolutios is 10 bit but y has truncated to 8 bit

Re: how to get analog-to-digital conversion accuracy??.

quimey — Fri, 16 Jan 2009 08:45:36 GMT

I saw this in a osciloscope ,the maximun and minimun value of the reference was for example 680mv and 300mv,wat is bandgap?

Re: how to get analog-to-digital conversion accuracy??.

denny_george — Fri, 16 Jan 2009 13:42:13 GMT

Hello Quimey,

Bandgap voltage is the refence voltage( =1.2 V) normally used calibrate the ADC and may be other voltage handling circuits (may be used outside MCU).Ok I see you don't have Bangap voltage cahannel in your MCU.

I assume you are using MC68HC908MR ( I caouldnot find HCS core version of MR family)

I see you have you have pins VREFH and VREFL popping out .What are its voltage levels ? Is it tied to analog VDD and VSS ?

You said " the mcu family y mr and the resolutios is 10 bit but y has truncated to 8 bit "

I guess you might have set the ADC reg for 8-bit truncation mode

Can you send the ADC driver code.Someone can give a look

Message Edited by Geo*** on 2009-01-16 05:51 AM

Re: how to get analog-to-digital conversion accuracy??.

quimey — Sat, 17 Jan 2009 20:23:28 GMT

ok, this is the code:

void init_adc(void){

ADSCR_ADCO=1;/*CONVERSION continua*/

ADSCR_ADCH0=1;/*ELIJO ATD1 PIN 63*/
ADSCR_ADCH1=0;
ADSCR_ADCH2=0;
ADSCR_ADCH3=0;
ADSCR_ADCH4=0;

ADCLK_ADIV2=1;/*ELIJO EL PRESCALER EN 8 PARA TENER APROX 0,5MHZ DE CLOCK DE ADC*/
ADCLK_ADIV1=0;/*8,9MHZ CRISTAL*/
ADCLK_ADIV0=0;

ADCLK_MODE1=0;/*ELIJO TRUNCAR A 8 BIT*/
ADCLK_MODE0=0;
ADSCR_AIEN=1;
}

regards

Thank you for your help

quimey