Hi all,

I'm trying to use the ADC on the ezDSP F2812 development board.
I made a simple voltage divider in a breadboard using resistors and 
then  buffered it with a voltage follower before sending it to the 
ADC pin for experimenting with the ADC. The breadboard is powered by 
a 3V3 voltage regulator. (It doesn't take it power from the board)

When i looked at the output values of the ADC while debugging in CCS, 
i saw that the output values of the ADC aren't 1101 and fluctuate btw 
1000-1300 for an input voltage of 812mV.

Another weird thing is, when i measured the voltage between the pins 
ADCINA0(P9-PIN2) and GND(P9-PIN1) while the circuit on the bread 
board is powered off, i saw 0.450V. Similarly, there were 0.730V btw 
these pins when i left them floating.(Weird)

I'm using the internal voltage reference and a slow ADC clock 
(2.5Mhz).

What could be reason of this?  I don't think that it is a software 
issue but i'm putting my code  below.

Thanks for your help.

/////////////////////////////////////////////////////
#include "DSP281x_Device.h"
#include "DSP281x_Examples.h"

int a=0;
int array[10];
void main(void)
{
	InitSysCtrl();
	InitPieCtrl();		
	InitPieVectTable(); 
	 
	InitAdc();			   	
   	// Configure ADC clock

        // Divide HSPCLK by 30 (75/30=2.5MSPS)
	AdcRegs.ADCTRL3.bit.ADCCLKPS = 15;	      	

   	// Configure ADC

        // Sampling Window = (ACQ_PS+2)/ADCCLK = 2us
	AdcRegs.ADCTRL1.bit.ACQ_PS = 3;	          
	AdcRegs.ADCTRL1.bit.SEQ_CASC = 1; // Cascaded Mode
   	AdcRegs.ADCTRL1.bit.CONT_RUN = 1; // Continuous run
   	AdcRegs.ADCTRL1.bit.CPS = 0;	  // Prescaler = 1	
   	AdcRegs.ADCMAXCONV.all = 0;       // 1 conversion sequences  
        AdcRegs.ADCCHSELSEQ1.bit.CONV00 = 0;      
        AdcRegs.ADCTRL3.bit.SMODE_SEL = 0;     // Sequental sampling
	AdcRegs.ADCTRL2.bit.SOC_SEQ1 = 1; // Start conversion sequence
	while(1)
	{	 	
		while (AdcRegs.ADCST.bit.INT_SEQ1 == 0); // Delay
		array[a] = AdcRegs.ADCRESULT0>>4;
		a++;
		if (a == 10) a=0;
		AdcRegs.ADCST.bit.INT_SEQ1_CLR = 1;
	}
}
///////////////////////////////////////////////////////////

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_____________________________________

 

hi,
  i would suggest that you should look at the ti manual for adc calibration
  there are two pins on the 2812 adc pins which produce 1 and 2 volts reference voltages
  use that and calibrate the adc
   
  also you can try a simple low pass filter/Kalman filter to stabilize your readings
  bye

Salih Cihan Tek <k...@hotmail.com> wrote:
          Hi all,

I'm trying to use the ADC on the ezDSP F2812 development board.
I made a simple voltage divider in a breadboard using resistors and 
then buffered it with a voltage follower before sending it to the 
ADC pin for experimenting with the ADC. The breadboard is powered by 
a 3V3 voltage regulator. (It doesn't take it power from the board)

When i looked at the output values of the ADC while debugging in CCS, 
i saw that the output values of the ADC aren't 1101 and fluctuate btw 
1000-1300 for an input voltage of 812mV.

Another weird thing is, when i measured the voltage between the pins 
ADCINA0(P9-PIN2) and GND(P9-PIN1) while the circuit on the bread 
board is powered off, i saw 0.450V. Similarly, there were 0.730V btw 
these pins when i left them floating.(Weird)

I'm using the internal voltage reference and a slow ADC clock 
(2.5Mhz).

What could be reason of this? I don't think that it is a software 
issue but i'm putting my code below.

Thanks for your help.

/////////////////////////////////////////////////////
#include "DSP281x_Device.h"
#include "DSP281x_Examples.h"

int a=0;
int array[10];
void main(void)
{
InitSysCtrl();
InitPieCtrl(); 
InitPieVectTable(); 

InitAdc(); 
// Configure ADC clock

// Divide HSPCLK by 30 (75/30=2.5MSPS)
AdcRegs.ADCTRL3.bit.ADCCLKPS = 15; 

// Configure ADC

// Sampling Window = (ACQ_PS+2)/ADCCLK = 2us
AdcRegs.ADCTRL1.bit.ACQ_PS = 3; 
AdcRegs.ADCTRL1.bit.SEQ_CASC = 1; // Cascaded Mode
AdcRegs.ADCTRL1.bit.CONT_RUN = 1; // Continuous run
AdcRegs.ADCTRL1.bit.CPS = 0; // Prescaler = 1 
AdcRegs.ADCMAXCONV.all = 0; // 1 conversion sequences 
AdcRegs.ADCCHSELSEQ1.bit.CONV00 = 0; 
AdcRegs.ADCTRL3.bit.SMODE_SEL = 0; // Sequental sampling

AdcRegs.ADCTRL2.bit.SOC_SEQ1 = 1; // Start conversion sequence
while(1)
{ 
while (AdcRegs.ADCST.bit.INT_SEQ1 == 0); // Delay
array[a] = AdcRegs.ADCRESULT0>>4;
a++;
if (a == 10) a=0;
AdcRegs.ADCST.bit.INT_SEQ1_CLR = 1;
}
}
///////////////////////////////////////////////////////////

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_____________________________________

 

I looked at the ADC input signals with a scope and it turned out to be 
a 50hz noise problem. I saw 200mV p-p 50Hz noise signal both in the ADC 
inputs and supply pins of the expansion connector. The source of this 
noise isn't the external circuit i connect, because it still exists 
even when i disconnect the breadboard from the DSP board. What is the 
source of this noise and how can i eliminate it?

Regards,

--- In c...@yahoogroups.com, chirag jagadish <chirag_jagadish@...> 
wrote:
>
> hi,
>   i would suggest that you should look at the ti manual for adc 
calibration
>   there are two pins on the 2812 adc pins which produce 1 and 2 volts 
reference voltages
>   use that and calibrate the adc
>    
>   also you can try a simple low pass filter/Kalman filter to 
stabilize your readings
>   bye
>

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_____________________________________

 

Hello again,

It seems like the noise was coming from the PC. When i disconnected the 
board from the PC and powered it while the circuit in the breadboard 
was connected to its ADC, the noise was gone. But there should be a 
solution that will make the board work properly while its connected to 
the parallel port of the PC, or should i run the application from the 
FLASH for using the ADC(i hope that this isn't the case).

Regards,

Check Out Industry's First Single-Chip, Multi-Format, Real-Time HD Video Transcoding Solution
for Commercial & Consumer End Equipment: www.ti.com/dm6467

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_____________________________________

 

hi,
  when you connect the 2812 to computer(PC), you can see the watch window and realize that
there is noise
  i do not understand how you could see the absence of noise when you cannot see the watch
window or you dont see the graph in ccs.
  can you tell me how you observe data without connecting to ccs.
  this might help me with my own project.
  thank you
  bye

Salih Cihan Tek <k...@hotmail.com> wrote:
          Hello again,

It seems like the noise was coming from the PC. When i disconnected the 
board from the PC and powered it while the circuit in the breadboard 
was connected to its ADC, the noise was gone. But there should be a 
solution that will make the board work properly while its connected to 
the parallel port of the PC, or should i run the application from the 
FLASH for using the ADC(i hope that this isn't the case).

Regards,

Check Out Industry's First Single-Chip, Multi-Format, Real-Time HD Video Transcoding Solution
for Commercial & Consumer End Equipment: www.ti.com/dm6467

You can post a message or access and search the archives of this group on DSPRelated.com:
http://www.dsprelated.com/groups/c28x/1.php

_____________________________________

 

I think he meant he is looking noice on Scope (not on watch windows).

And I think noise is coming into circuit due to lack of Isolation.
Try to use optical isolation to reduce noise.
Thanks

Viks

On Feb 19, 2008 9:10 AM, chirag jagadish <c...@yahoo.co.in> wrote:
> hi,
> when you connect the 2812 to computer(PC), you can see the watch window and
> realize that there is noise
> i do not understand how you could see the absence of noise when you cannot
> see the watch window or you dont see the graph in ccs.
> can you tell me how you observe data without connecting to ccs.
> this might help me with my own project.
> thank you
>
> bye
> Salih Cihan Tek <k...@hotmail.com> wrote:
> Hello again,
>
> It seems like the noise was coming from the PC. When i disconnected the
> board from the PC and powered it while the circuit in the breadboard
> was connected to its ADC, the noise was gone. But there should be a
> solution that will make the board work properly while its connected to
> the parallel port of the PC, or should i run the application from the
> FLASH for using the ADC(i hope that this isn't the case).
>
> Regards,
Check Out Industry's First Single-Chip, Multi-Format, Real-Time HD Video Transcoding Solution
for Commercial & Consumer End Equipment: www.ti.com/dm6467

You can post a message or access and search the archives of this group on DSPRelated.com:
http://www.dsprelated.com/groups/c28x/1.php

_____________________________________

 

Hello again,

I did not observe the "data" when the 2812 was disconnected from the 
PC.

Initially, i was only observing the values in the watch window and 
saw that there is a fluctuation problem. Later, when i started 
looking at the analog signal that is applied to the ADC pin with an 
oscilloscope, i saw that there is a 50Hz noise signal coupled to the 
ADC input with a peak-to-peak value of approx 200mV. After that, i 
stopped looking at the watch window and disconnected the board from 
the printer cable. When i looked at the same signals again with the 
scope, the noise was gone. 

If i'm not mistaken, the reason for the fluctuation is this 50Hz 
noise which is coupled to every signal both in the DSP board and in 
my circuit on the breadboard, while the 2812 is connected to the 
parallel port via the printer cable. The parallel port pins have this 
50Hz signal on them even when nothing is connected to them. 
Unfortunately, i don't know how to get rid of this noise. I would be 
very much appreciated if someone would tell me the solution. 

By the way, if i would need to observe the ADC data while the board 
isn't connected to the PC, i would write the value to the GPIO pins 
and connect these pins to an external DAC.
Regards,

--- In c...@yahoogroups.com, chirag jagadish <chirag_jagadish@...> 
wrote:
>
> hi,
>   when you connect the 2812 to computer(PC), you can see the watch 
window and realize that there is noise
>   i do not understand how you could see the absence of noise when 
you cannot see the watch window or you dont see the graph in ccs.
>   can you tell me how you observe data without connecting to ccs.
>   this might help me with my own project.
>   thank you
>   bye
>   
> 
> Salih Cihan Tek <korgeneral297@...> wrote:
>           Hello again,
> 
> It seems like the noise was coming from the PC. When i disconnected 
the 
> board from the PC and powered it while the circuit in the 
breadboard 
> was connected to its ADC, the noise was gone. But there should be a 
> solution that will make the board work properly while its connected 
to 
> the parallel port of the PC, or should i run the application from 
the 
> FLASH for using the ADC(i hope that this isn't the case).
> 
> Regards,
>

Check Out Industry's First Single-Chip, Multi-Format, Real-Time HD Video Transcoding Solution
for Commercial & Consumer End Equipment: www.ti.com/dm6467

You can post a message or access and search the archives of this group on DSPRelated.com:
http://www.dsprelated.com/groups/c28x/1.php

_____________________________________