AVR-Chat

Hi Fuentes.

>I tried to improve the accuracy by lowering the adc
>clock:
> ADCSRA = 0xE0 | 0x07;//set last 3 bits
>that was to set bits ADPS2, ADPS1, ADPS0. Doing that
>the prescaler is set to use a division factor of 128.
>But it doesn't make the ADC more stable. Actually, it
>makes it less accurate, do you know why?

Yes this is because you charge the capacitors inside the ADC with the outside manipulation and when you convert at high speed you will be able to read the value of the capacitor almost instantaneously BUT if you read it at a slower speed it will begin to discharge and you will loose precision.

Best regards

L. Madsen

I solved some hardware problems and now the ADC is
more stable. This is the source code:

void init_adc(void)
{
ADMUX |= (0x01 << 6);
ADMUX &= (~ (0x01 << 7));
ADCSRA = 0xE0;
DDRA = 0x00;
}

unsigned int read_adc(void)
{
int adc;
adc = (ADCW & 0x03FF);
return adc;
}

I tried to improve the accuracy by lowering the adc
clock:
ADCSRA = 0xE0 | 0x07;//set last 3 bits
that was to set bits ADPS2, ADPS1, ADPS0. Doing that
the prescaler is set to use a division factor of 128.
But it doesn't make the ADC more stable. Actually, it
makes it less accurate, do you know why?

Jose




--- Dave VanHorn
>
>
> >
> >I get a rock steady ADC reading using AVR chips
> with 2 resistors on the input.
> >That is without sleep mode or an LC filter. That is
> using a 115.2KHz ADC
> >clock.
> >Anything lower than 200KHz should be fine.
>
> Indeed.
>
> There are constraints to be observed, like when you
> change channels, and
> making sure that your Vref is quiet.
>
> You should filter the input though, no sense having
> anything on the input
> that is significantly faster than your sample rate,
> or even any where near
> your sample rate.
> I get a couple of bits dither, but the systems that
> I'm working in aren't
> really designed for low noise, nor do they care.
> The upper 8 bits is
> plenty, so I didn't bother cleaning up the stuff
> that's below the noise floor.
>
>
>
>
>
> Yahoo! Groups Links
>
>
> AVR-Chat-unsubscribe@yahoogroups.com
>
>
>
>
>
>



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