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DAC8574: DAC8574 Voltage reference issue

Andrew Cserna
Prodigy 55 points
Part Number: DAC8574
Other Parts Discussed in Thread: LM4040

Dear Guys @ TI!

We made a simple circuit with your DAC8574 DAC. When I shunt the lower (U19) diode - so the VrefL goes to GND, the circuit works (almost) fine  (on all 4 channels). In this case the circuit pulls the refH voltage to about 2.8V.

But with this original configuration - even when I put 0x0000 to the DAC register, the output is about VDD (3.3V), which is higher than the VrefH.

Could you please help me what can the problem be?

Best regards:

András Cserna

  • 0
    TI__Guru 55213 points

    Hi Andrew,

    I think the root of the problem is the two resistors you have connected to your zener diodes (R158, R157).  Looking at the datasheet for the DAC8574 we list the reference input current as typically 80µA, with a max of 120µA.  If you take that into account you will see that the 4.7kΩ (R158) has about 380mV dropped across it.  This might explain the 2.8V you measured.  The zener diodes also require some amount of current to reach their rated voltage, you could find that in the datasheets for the diodes.  If you remove the DAC from the circuit and measure the reference voltages I think you would see that they are a bit lower than expected.  You should expect somewhat better performance by lowering the resistor values so that each zener has about 5mA available to it (check the zener's datasheet for what they recommend).  

    In general, you would see better performance using a voltage reference for VREFH and VREFL.

    Please let me know if you have any more questions.

    Paul

    DAC Applications 

  • 0 in reply to Paul_Frost
    Prodigy 55 points

    Dear Paul!

    First of all, thanks for your fast and kind response.

    Im a bit confused - even if I would use the references that you suggested, Im not sure how to set the circuit for proper operation.

    (BTW Originally the "zeners" are Ti LM4040 ). I think I understand what you said about the high reference - (The voltage dops on the high series resistor - ~380mV @ 80-100µA). But im not sure how to set the low side.

    here are some additional measurement I made with my circuit:

    ---------------------------

    If I remove the references, the circuit works as expected- But I'd like to have 16bit accuracy between 2 and 3V ('cause it sets the ref voltage of a precision circuit (based on ADL5317)) Thats why Id like to set the ref's properly.

    ----------------------------------------------

    If I put a DC power supply to the VrefL like this:

    The ADC somehow "forces" the VrefL to be about 0.85V (so if I set the supply for example 0.5V the Vref remains the same)

    VrefL[V] DAval Vout[mV]
    0.87 0x0000 1730
    0.87 0xCC90+ 3240 ("saturation")
    1.63 0x0000+ 3250 ("saturation")

    I dont even get where the 2x factor comes from? And it looks like the Href has no point.

    ----------------------------------------------------

    VrefH removed - does nothing (Vout = VDD all the time)

    -------------------------------------------------------------------

    VrefL[V] DAval (register value) Vout[mV]
    0.00 0x0000

    458
    0.00 0xEC70+ 3240 ("saturation")
    1.00 0x0000

    1994
    1.00 0x8E50+

    3240 ("saturation")

    1.20

    		 0x0000 2390
    1.20 0x6C20+

    3240 ("saturation")

    ---------------------------------------------

    I also tried with some AA batteries :)

    ---------------------------------------------

    conclusion:

    - if i use the voltage references that you suggested, do I need to take into account the 2x gain (and why is it there)

    -Do I need to provide additional current paths (like the 470ohm in the last pic) parallel with the ref IC

    regards:

    A.Cs.

  • 0 in reply to Andrew Cserna
    TI__Guru 55213 points

    Andrew,

    I have sent you a request to connect on the e2e forums.  Please accept the request and I can sent you a message offline to get this resolved.

    Thanks!

    Paul