PIC16F887 and Anti-Aliasing..

[@m@]

hello dear friends..
I am working on PIC16F887 , when i applied AC signal to its ADC input pin then it is not giving a correct conversion for sine wave ...
So how to select best sampling rate for A/d conversion .. ?
here is my code I changed and implement different values of ADCON0 but no use...

unsigned int temp_res;
 char buf2[6];
 int i;
void main() {
  UART1_Init(9600);
  ADCON1 = 0x80;
  ADCON0=0xC1;
  TRISA  = 0xFF;
  ANSEL  = 0xFF;              // Configure AN2 pin as analog
  //ANSELH = 0;                 // Configure other AN pins as digital I/O
  C1ON_bit = 0;               // Disable comparators
  C2ON_bit = 0;

  TRISA  = 0xFF;              // PORTA is input
  TRISC  = 0;                 // PORTC is output
  TRISB  = 0;                 // PORTB is output

  do {
    temp_res = ADC_Read(0);   // Get 10-bit results of AD conversion
    PORTB = temp_res;         // Send lower 8 bits to PORTB
    PORTC = temp_res >> 8;    // Send 2 most significant bits to RC1, RC0
  wordToStr(temp_res,buf2);

    for(i = 0; i < 6; i++)
    {
      UART1_Write(buf2[i]);
      Delay_ms(80);
      }} while(1);
      }

here is my aliased sine wave is attached
I have already pass my signal through clamper and then anti-aliasing filter..
But what i think so that problem is with MCU frequency clock ..When I changes it to 150 Mhz so signals becomes much better but it is too high ..
Please guide...

 

[ericgibbs]

hi,
What range of input frequencies are you working with.??

Also can you post a diagram for clamp and anti alias filter.?

 

[@m@]

Yes , Sure .. here it is...

 

[Pommie]

Do you realise that you are only reading 10 samples per second? If you are sampling 50Hz then you need to be reading the ADC at a much higher rate.

Edit, just looked at your code again and you're reading at about 2 samples per second.

Mike.

 

[@m@]

Yes.. I know that there is problem with my code especially when Input is AC signal because this code works fine for DC input voltages...
I am newbie to PIC and and made this program with the help of different examples and data sheet .. So please could u guide that which code lines should be edited for my sampling ..
I put and change values in ADCON0 register but not change ..
Please guide... thank u ...

 

[Pommie]

Assuming you don't need to sample continuously, I would sample into a buffer and then send to the PC. Your attachment above suggests you are sampling a 5Hz wave form, is that correct? Which compiler are you using and can you post a compilable version of your code. I ask as at the moment I have no idea whether you are using library functions or not and what they do.

Mike.

 

[@m@]

I am using the MiKroC compiler..
Yes in the above attachment i was sampling 5Hz sine wave..
I edited this code using the example available in MikroC for ADC and UART...

 

[Pommie]

I don't use MikroC and so I don't know what there libraries do. However, to get higher res just change the delay and see how that works out.

  do {
    temp_res = ADC_Read(0);   // Get 10-bit results of AD conversion
    PORTB = temp_res;         // Send lower 8 bits to PORTB
    PORTC = temp_res >> 8;    // Send 2 most significant bits to RC1, RC0
    wordToStr(temp_res,buf2);
 
    for(i = 0; i < 6; i++)
    {
      UART1_Write(buf2[i]);
      Delay_ms(2);
     }
  } while(1);
}

This should allow about 60 to 70 samples per second.

Edit, looks like the board is broke again and not showing colours. In the above I changed the delay to 2mS.

Mike.

 

[@m@]

u changed delay to 2 ms.. ? rite...

 

[Pommie]

Yes, at 9600 baud 2mS is long enough.

Mike.

 

[@m@]

hmmm, Something different but it is not sine.. huh.. ..

 

[Pommie]

That is a very clipped waveform. It looks like the AC that you are feeding the chip is greater than 5V.

Mike.

 

[@m@]

okay,, Thanks Alot... See my waveform I think it is good now... ?
But i think it is still not clamping correctly ..

 

[Pommie]

It now appears to have an amplitude of 2.5V and is going negative.

Mike.

 

[@m@]

Now, I just changed the clamping circuit...
Input is 2.4 V Amplitude Sinewave