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DAC8801: Adjustable IVC

Fabian Brings
Prodigy 40 points
Part Number: DAC8801

Greeting everybody,

I am looking to design an adjustable current-to-voltage converter circuit. The basic circuit is just a regular Op-Amp style IVC, where a feedback resistor mediates the conversion. Since my design should run on 3.3 V dual supply and be quite compact I was trying to use an mDAC device. However I am getting some unexpected results. The mDACs Vref - Iout path replaces the feedback resistor on the Op-Amp. If I leave the input of the converter open (aka. 0 A flowing in), I would expect an output voltage of 0 V. However, it seems that the Op-Amp still needs to provide a voltage to keep the negative input at 0 V. This voltage is furthermore depending on the code that the mDAC is set to.

My thoughts were, that the mDAC is an R2R ladder and as such mostly behaves like a resistor. Seemingly that notion was wrong and so I wanted to ask, if there is something I am not seeing or not aware of, or if such a configuration can simply not work and why.

Thanks in advance,

FB.

  • 0
    TI__Mastermind 46290 points

    Fabian,

    Can you please share a schematic for your design?

    As you have eluded to, MDACs generally expect a transimpedance amplifier configuration at their IOUT pin utilizing the internal RFB feedback resistor. The use of the feedback resistor is important as that internal resistor is ratiometrically matched to the R-2R ladder and thereby delivers the expected VREF defined output voltage.

    Hopefully your schematic resembles Figure 18 on page 8 of the datasheet.

  • 0 in reply to Kevin Duke
    Prodigy 40 points

    Hello,

    thank you for the fast reply! Of course my schematic does not look like in the datasheet at all! What I am planning to do was to use the R2R ladder in the mDAC as an adjustable resistor to use as a feedback resistor for the current-to-voltage converter. Something like this:

    In principle I don't see why this should not work, but there seems to be a code-dependent voltage drop inside the DAC. I know that this is not the usual way to emply this circuit, but I would like to understand what might be causing my problems to find a solution to make it work!

    Greetings and have a nice day,

    FB.

  • 0 in reply to Fabian Brings
    TI__Mastermind 46290 points

    Hello Fabian,

    Thank you for clarifying the setup.

    Fundamentally I think you should expect a voltage drop in this configuration - that is more or less how the trans-impedance amplifier configuration functions.

    Page 8 shows you the internal construction of the R-2R ladder. It is a binary weighted current divider when looking from VREF to IOUT. If you instead look from IOUT to VREF it is a binary weighted voltage divider. So, the direction of VREF and IOUT does make a difference in the design.

  • 0 in reply to Kevin Duke
    Prodigy 40 points

    Thank you Kevin,

    you are right in that I oversaw the fact that it is not acting like a simple adjustable resistor, but like an adjustable divider. I will check my calculations and compare my results again.  Then I will let you know if I am getting the expected output.

    Greets,

    Fabian.

  • 0 in reply to Fabian Brings
    TI__Mastermind 46290 points

    Hello Fabian,

    I am glad that this is helpful. Without having any further details on your observations I think the issue is the orientation of the VREF and IOUT pins in your circuit. Having a programmable voltage divider versus current divider will make a difference. The current divider basically gets you a programmable resistor, while the topology you have right now will yield different results.

    If this thread goes longer than 2 weeks without a reply, our forum tools will automatically lock it. But, you can still ask another question or click "ask a related question" and either myself or someone else on my team will continue to support you. Until then, I'm going to mark this as resolved so it does not potentially linger as open indefinitely.

  • 0 in reply to Kevin Duke
    Prodigy 40 points

    Hello Kevin,

    the issue was indeed cause by a design flaw on my end. The schematic I showed actually omitted, that I use the non-inverting input of the OP to bias the current input. This however causes a problem, because the DACs internal ladder is always referenced to ground. If the non-inverting input is at 0 V the circuit works as expected, but not otherwise. I am changeing the design, to use an mDAC that exposes the reference line of the ladder and solve the problem this way.

    Greets and thanks a lot for your help,

    Fabian.