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ADS7813 resolution problem

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Posted by Robert Katona

on Mar 22 2012 14:59 PM

Hi there!

I had ordered a sample of ADS7813 and tried to get work with an Arduino board. And my problem is i cant get out a better resolution than two decimal place. The first 10 bit seems ok, from MSB, but the last 6 bit is bouncing pell-mell. Here are some conversion results:

3.2569353	10100110101110
3.2569353	10100110110101
3.2584612	10100110110110
3.2599871	10100110111001
3.2609026	10100110110101
3.2609026	10100110111000
3.2609026	10100110110001
3.2657856	10100110110100
3.2599871	10100110110100
3.2566301	10100110110001
3.2526626	10100110110100
3.2566301	10100110110101
3.2593767	10100110101100
3.2609026	10100110110110

The first column values is in voltage the second columns values is the conversion result.

I bulid the circuit in a bread-board i used the same capacitors as in the datasheet i tried another input values and i got the same results.

here is my code in Arduino (same as C):

digitalWrite(CLOCK,LOW);

digitalWrite(CONV,LOW);

         //this start the conversation

delayMicroseconds(1);

digitalWrite(CONV,HIGH);

while(busy==0){                               // while BUSY is high still wait

busy=digitalRead(BUSY);

}


for(int i=0;i<16;i++){                  //here start the read of the 16 bit

digitalWrite(CLOCK,HIGH);

x[i]=digitalRead(DATA);

delayMicroseconds(20);

digitalWrite(CLOCK,LOW);

}


for(int j=1, c=16384j<16;j++,c=c/2){                   //this is how i piece together the 16 bit data in decimal, 15bit because the firs is the sign bit

val+=x[j]*c;

}

val=5*val*(double)1/32767; //this is how i convert the number to voltage with +-5V input range

What did i mess up?

Cheers

Robert

All Replies

Posted by Tom Hendrick

on Mar 24 2012 13:41 PM

Guru86220 points

Hi Robert,

Could you possibly post a schematic showing your connections to the ADS7813? The 'C' code is really not going to impact the last 6 LSBs toggling, that would be caused by you power, reference and/or the actual analog input circuitry.

Regards,

Tom

Posted by Robert Katona

on Mar 24 2012 15:39 PM

Prodigy35 points

Here is the wiring diagram:

I used the USB power i dont use any adapter. I tried a battery in place of the potentiometer and i got a same result.

Best regards

Robert

Posted by Robert Katona

on Apr 02 2012 13:59 PM

Prodigy35 points

Hi again,

So no one can help me?

Robert

Posted by Tom Hendrick

on Apr 04 2012 14:15 PM

Verified Answer

Verified by Tom Hendrick

Guru86220 points

Hi Robert,

Take a look over these articles:

Designing SAR ADC drive circuitry, Part I: A detailed look at SAR ADC operation. Analog Zone. Download at: http://www.analogzone.com/acqt0221.pdf

Designing SAR ADC drive circuitry, Part II: Input behavior of SAR ADCs. Analog Zone. Download at: http://www.analogzone.com/acqt1003.pdf

Designing SAR ADC drive circuitry, Part III: Designing the optimal input drive circuit for SAR ADCs. Analog Zone. Download at: http://www.analogzone.com/acqt0312.pdf

Regards,

Tom

Posted by ism aaa

on May 28 2012 05:32 AM

Prodigy125 points

Hi Robert, can you please help me about using ADS7813 with arduino?

I try to use these codes but it dont work, i need immediately help.

thanks.

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ADS7813 resolution problem