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DAC 8760 BOOST CONFIGURATION FOR IOUT

Rajesh Rajesh
Intellectual 500 points
Other Parts Discussed in Thread: DAC8760

hi sir,

i had connected the circuit as given in datasheet at page 41 BOOST CONFIGURATION FOR IOUT.

transistor BC547 used.

i gives a non linearity current output for loads.

for 250 ohm reistance load. 

for 500 ohm.

here i have attached the output graphs from scope , kindly find the attached.

please provide a guidance for this 

thanks and regards.

rajesh

  • 0
    TI__Mastermind 46290 points

    Rajesh,

    Could you please share a complete schematic of this design? Is this a purely resistive load or is there some additional capacitance/inductance? Is this capture a single code-to-code transition or are you applying a ramp (i.e. sequentially stepping through all codes)?

  • 0 in reply to Kevin Duke
    Intellectual 500 points

    hi sir ,

    i had used the circuit as like as given in datasheet for your reference i have given below

    cir.pdf

    i had used a 1/4W  resistor as load (200 ohm , 500 ohm)

    i had creating a ramp in code as given below.

    <code>

    while(1)
    {

    DAC8760write(DAC8760_ADD_WRITE_TO_DAC_REG,dacValue++);
    __delay_us(5);
    // unsigned char sr =DAC8760read(DAC8760_ADD_READ_STATUS);
    // printf("the vsale status =%x \n" ,sr);

    if(dacValue > 0Xfffe)// DAC8760CalStatus.maxCount)//13107) //DAC8760CalStatus.maxCount)
    {
    dacValue =0;
    }
    __delay_us(10);

    }

    </code>

    regards 

    Rajesh

  • 0 in reply to Rajesh Rajesh
    TI__Mastermind 46290 points
    Rajesh,

    I understand that your schematic resembles what is shown in the datasheet, but seeing the actual schematic to double-check the design would be helpful. It also allows me to look at other parts of the design to try to understand what is happening here.

    It would be helpful to know what supply you are using in the design. Without knowing the supplies I can't tell you for sure if this is a valid concern or not, but one item to check is if the boost transistor is generating excessive heat. I see thermal resistance of 200C/W for the BC547 which is pretty high. There's no indication in the BC547 datasheet on whether this assumes a particular thermal relief scheme or not so there's a possibility, depending on your layout, that it's actually worse. If this transistor is heating up it could be causing other components to drift.

    Can you disable/bypass the boost circuit and test the current output using the internal transistor? It would be nice to isolate the boost circuit itself from the rest of the design and your test procedure.