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DAC60501: It is not possible to get a current less than ~4.65 mA.

Nikolay Puchkov
Prodigy 40 points
Part Number: DAC60501
Other Parts Discussed in Thread: DAC7571, ISO1541, XTR116, XTR115

Hi. A strange effect occurs with the DAC60501. It is not possible to get  current less than ~4.65 mA. The graph shows what it looks like. An electrical diagram is also attached. Measurements are made at the OUT terminal. How can this be and what is my mistake?

REF_PWDN=1
DAC_PWDWN=0
REF-DIV=1
BUFF-GAIN=1

The previous one was DAC7571 and there were no such effects.

  • 0 in reply to Nikolay Puchkov
    TI__Mastermind 19086 points

    Hi,

    Can you please share the code vs DAC voltage output rather than current output values from XTR?

    I need to check whether DAC is giving proper output for each code. Also whats the power supply for DAC?

    Regards,

    AK

  • 0 in reply to Akhilesh K
    Prodigy 40 points

    Hi AK. I will get the exact dependence of the voltage on the code and post it a little later.

    The idea was as follows:
    Сode 0x800 gives 0.8V. Current before XTR 0.8V/20k=40mkA. Then 40mkA*100(XTR)=4mA.
    Code 0xFA0 gives 4V. 4/20k=0.002mA and 0.0002mA*100=20mA. OK 4-20mA.
    Above code ~0x900 works perfectly.

    The power supply is organized as follows:
    External +24V is supplied to XTR116(pin7). pin8: +5V is the power supply for the DAC60501 and ISO1541. pin1: 4.096V is the Vref for the DAC.

    Regards

  • 0
    Intellectual 420 points


    From the XTR116 data sheet:

    MINIMUM-SCALE CURRENT

    The quiescent current of the XTR115/6 (typically 200µA) is the lower limit of its output current. Zero input current (IIN = 0) will produce an IO equal to the quiescent current. Output current will not begin to increase until IIN > IQ/100. Current drawn from VREF or VREG will add to this minimum output current. This means that more than 3.7mA is available to power external circuitry while still allowing the output current to go below 4mA.

    The DAC60501ZDQ is 1.5mA typ, 2.0mA max.

    The ISO1541 is 2.5mA typ, 3mA max on VCC2 if the clocks idle high which avoids extra current through the R19 pack.

    It looks like your quiescent currents are just too high to be accommodated by the 4-20mA current loop transmitter down to the 4mA minimum.

  • 0 in reply to Nikolay Puchkov
    TI__Guru 53633 points

    Hi Nikolay,

    It is also critical to ensure that the loop supply is not referenced to the board's GND(Iret).  The loop- node will always be a lower potential than ground.  If they are shorted, the current loop will not be able to regulate.

    In addition, are you disabling the DAC60501's internal reference? If the references are shorted, you could be drawing additional current.

  • 0 in reply to John Handwork
    Prodigy 40 points

    Hi, John. You're really right. I changed the DAC in the working circuit, but I forgot to calculate the increased current consumption. By consuming more than 4mA I will not reach the exact 4mA output. I need to review the scheme.

    Regards,
    Nikolay.