DAC8728 has strange behavior while converting

hi Eugenio thx for that fast reply,

alright i will atach a scrennshot of the dac schematic...hope thats ok

some explains to the schematic... HVDD is 15V, HVSS is -15V, DVDD is 3.3V,

in addition i changed  the voltage devider of the RSTSEL-pin so there apply 3.3V

what do you mean with the power pad i ?? is it named different in the data sheet? i can't find it?

thanks stricker

Stricker,

ser str said:

what do you mean with the power pad i ?? is it named different in the data sheet? i can't find it?

What I meant by "power-pad" was "thermal-pad". As you can read in page 10 of the datasheet, it is recommended that you connect it to AVSS or leave it floating.

Thank you for sharing the schematic. It seems like everything is correctly connected.

1. Are you following the power-on sequence specified on page 41 of the DAC8728 datasheet?
   1. This would be my biggest suspect. The power-on sequence is very important. If it is not used correctly, the internal registers of the device will not power-on correctly and can cause the device to behave abnormally.

2. Are you using the package with the thermal pad? What voltage do you have the thermal pad connected to?
   1. This can sometimes cause strange behaviors if not connected correctly
3. Have you verified that the voltage at RSTSEL is 3.3V?
4. Can you share a scope capture of the SPI interface?

alright eugenio,

first i got a scope picture of my start sequence for your , please have a look on it...

ch1 = DVDD & IOVDD

ch2 = AVDD

ch3 = AVSS

ch4 = VREF

Hi Eugenio,

well i've changed my circuit and my start sequence looks like that now...

but the strange behavior is still existing !

ok...then i've put some pull up resistors on the ~CLR & ~LDAC pins
 ----> but nothing changes !

then i've made a software loop for one output-chanel and gave out every single data-bit....(i've forgot to make that picture with the osci...and will be able to make it next tuesday! ) but i've noticed that when the bits in the middle ( DB5 - DB12) are applying nothing happens...the other are alright.

I mean i can meassure the 3.3V applying at the DAC-pin directly but  nothing happens at the output!

have you some another ideas...i'm a little bit frustrated.

thaks for helping

Hi Stricker,

I understand your frustration. There is nothing worse than debugging a circuit in a lab for hours at a time with no progress!

Don't worry we will solve this issue.

ser str said:

well i've changed my circuit and my start sequence looks like that now...

The ramp looks much better now. It still worries me that the digital ramp has a long step at ~0.5V. I've seen this cause problems in other devices, so I would suggest attempting power-on using a cleaner ramp.

ser str said:

then i've made a software loop for one output-chanel and gave out every single data-bit....(i've forgot to make that picture with the osci...and will be able to make it next tuesday! ) but i've noticed that when the bits in the middle ( DB5 - DB12) are applying nothing happens...the other are alright.

It would be useful to get a few captures of the digital interface.

And, answering your question, yes these devices are tested after they are fabricated. To my knowledge the distributors do not test the devices themselves.

allright next time i'll be at the firm i wil send you some captures....and i will change the start sequece ....thx for your patience

Hi Eugenio,

alright i got the captures a little bit earlier...well please take a look at them

1. this capture shows every bit from 1-16 with +/-15 V supply and default offset

2. the 2nd one has the same settings exept an offset of zero...

the 3rd and the 4th have a loop wich gives out values from 0 - 65535...

3.  default offset

4. offset of zero

perhaps now you have some ideas whats wrong

thx a lot

hi eugenio,

did you looked into the scope pictures?

what do you think about them???

best regards stricker

yes of course...here we are. this is the msp430F5529 the red boxes are the data & adress bits...don't be scared...i had to connect them like this because the project was began by another guy and

now i have to shfit the adress & data bits together... : (

and here some code lines...

//set dafault dac values

set_DA_adress(0x0);
    P4OUT &= ~BIT3;                        //R/~W = 0
  
    DA_value(0x8000);                    //
    P4OUT &= ~BIT2;                        //~CS = 0
   
    P4OUT |= BIT2;                        //~CS = 1
    P4OUT |= BIT3;                        //R/~W = 1

....

do {
          
            for(i=0; i<65535; i++){

                    set_DA_adress(DAC_0);                                //DAC adress = 8
                    P4OUT &= ~BIT3;                                        //R/~W = 0
                   
                    DA_value(i);
                  
                    P4OUT &= ~BIT2;                                        //~CS = 0
                   
                    P4OUT |= BIT2;                                        //~CS = 1
                   
                    P4OUT |= BIT3;                                        //R/~W = 1

 P4OUT &= ~BIT1;                                        //toggle ~LDAC ---> gleichzeitige aktualisierung der DAC-ausgänge
               
 P4OUT |= BIT1;
               
        }while(1);

..........

and here are the shift operations first DA_value and second adress_value

void DA_value(unsigned int displValue){                          

  
    // MSB                                                                                                                                           LSB
    //    15    14      13     12      11    10       9       8       7        6        5      4       3        2       1       0
    //P 3.7    3.6    3.5    3.2    3.1    3.0    2.7    2.6    2.5    2.4    2.3    2.2    2.1    2.0    1.7    1.6
  

    unsigned int help = displValue;
    unsigned int copyPort = PAOUT_L;

    help <<= 6;
    help &= 0xC0;
    copyPort &= ~0xC0;
    copyPort |= help;
    PAOUT_L = copyPort;

    help = displValue;
    copyPort = PAOUT_H;

    help >>= 2;
    help &= 0xFF;
    copyPort &= ~0xFF;
    copyPort |= help;
    PAOUT_H = copyPort;

    help = displValue;
    copyPort = PBOUT_L;

    help >>= 10;
    help &= 0x7;
    copyPort &= ~0x7;
    copyPort |= help;
    help = displValue;
    help >>= 8;
    help &= 0xE0;
    copyPort &= ~0xE0;
    copyPort |= help;
    PBOUT_L = copyPort;

void set_DA_adress (int DA_chanel){          
    // Adress: A4    A3    A2    A1    A0          

    //         P    8.1    8.0    7.7    4.7    4.6
    //            {-------out--------}
    int help = DA_chanel;
    int copyPort = PBOUT_H;

    help <<= 6;
    help &= 0xC0;
    copyPort &= ~0xC0;
    copyPort |= help;
    PBOUT_H = copyPort;

    help = DA_chanel;
    copyPort = PDOUT_L;

    help <<= 5;
    help &= 0x80;
    copyPort &= ~0x80;
    copyPort |= help;
    PDOUT_L = copyPort;

    help = DA_chanel;
    copyPort = PDOUT_H;

    help >>= 3;
    help &= 0x3;
    copyPort &= ~0x30;
    copyPort |= help;
    PDOUT_H = copyPort;

i've checked the shift-op several time and they seems right....

thanks for helping

ohhh sry i understanded that in wrong way...

i will send you the scopes as soon as possible (think it will be on tuesday)....