Here is the code that I have written based on filter feedback.  I think the
straight average gives a better result. I am using a 877 and seem to get +/-
1.5bits of random noise without filtering (input is pot tied to +5V, also
Vref+ tied to +5V).  Is there a better way?  Are there any good sites
covering this subject?


                ADC_FILT = 0;                   // Clear out previous count
                for (i=0; i<64; i++)
                        {
                        ADIE    = 0;            // Masking the interrupt
                        ADIF    = 0;            // Resetting the ADC interupt bit
                        ADRESL  = 0;            // Resetting the ADRES value register
                        ADRESH  = 0;
                        ADGO    = 1;
                                                // Staring the ADC process
                        while(!ADIF) continue;  // Wait for conversion complete

                        ADC_VALUE = ADRESL;     // ADC registers are split 2 and 8 bit
                        ADC_VALUE += (ADRESH << 8);     // Combined into a single integer

                        ADC_FILT += ADC_VALUE;
                        }
                        ADC_FILT = (ADC_FILT/64)+0.5;   // Average of 64 readings to reduce noise

//                      ALTERNATIVE FILTER.  Change i to <255 in the for statement above
//                      Comment out 3 lines below and replace with 2 lines below.
//                      Adjust delay in analog1.c
//
//                      Software low pass filter to attenuate noise by -12dB
//                      ADC_FILT = ADC_FILT + ((ADC_VALUE - ADC_FILT) * 0.0625);
//                      }

                        if (ADC_Channel == 0) ADC_VALUE = ADC_FILT*4.83;// Scaled to read 0 - 5V
                        else ;                  // All other channels un-scaled
                        ADC_VALUE = bintobcd (ADC_VALUE);       // convert to BCD
                        return (ADC_VALUE);             // Return the value of the ADC process

Ian

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> -----Original Message-----
> From: pic microcontroller discussion list
> [mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Mike Mansheim
> Sent: 27 August 2002 17:26
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: [PIC]: Real newbie question on ADC
>
>
> >>> FILT <-- FILT + (NEW - FILT) * FF
>
> >> Using your formula and simulating in Excel I always get an offset
> >> dependant on FF...<snip>
>
> > The formula was not intended for just integers.  It is usually a good
> > idea to have enough fraction bits for the bits of shifting implied by
> > FF... <snip>
>
> If you're not familiar with fraction bits (newbie reference in subject),
> that just means you carry the working data in the upper bits of a longer
> variable.  For example, if you want to use the full 10 bits of the a/d,
> you can use a 16 bit variable and use the bottom 6 bits as fraction bits.
>
> An example, in C, that shows how this works:
> (the variables are 16 bit unsigned integers)
>
> // read_a2d() is some function that puts 10 bit a/d reading in NEW
>
> // shift new a/d reading to the top of the word
> NEW = read_a2d() << 6;
>
> // this is for FF = 1/8
> FILT = FILT + (NEW/8) - (FILT/8);  // this particular rendition of
>                                     // the formula does the divide
>                                     // twice, but lets you completely
>                                     // ignore signs
>
> then if you need to use the filtered reading for anything, pick off the
> top ten bits of FILT.  Say you have a display function that takes a 16
> bit variable as a parameter:
>
> display(FILT >> 6);   // (but don't use FILT = FILT >> 6; to work with
>                       //  it; you'll lose the fraction bits
>
> As Olin pointed out, if you use an 8 bit a/d reading and 8 fraction bits,
> the bit shifting in this example simply becomes a matter of selecting
> high and low bytes.  And if FF = 1/256, then the divide(s) get real
> simple too!
>
> Hope this helps.
>
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