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Need help picking a Multiplying DAC (I think)

lslonim
Expert 1710 points
Other Parts Discussed in Thread: DAC8811, TINA-TI

Hi,

I have kind of a strange product to design, which I think starts with a multiplying DAC.

I'm given a DC input voltage somewhere in the range of +5 to +10V. When the input is +10V I have to put out a programmable voltage which can range from -60mV to +60mV which is scaled from the DC input voltage. In other words, if the DC signal drifts a little, my scaling needs to drift with it, and if it drops to 5V the output would be half of what it was at 10V. The code value written to the DAC sets what the output voltage is.

I need resolution on the output of about 1uV. The output change rate can be very slow, 1Hz update rate for example.

My thinking was to scale the input down before going into the DAC since otherwise I'm wasting a lot of the range.

Can you point me in a direction to start looking? Are there DACs that can multiply with such small input voltages?

Thanks,

Lloyd

  • 0
    TI__Guru 53773 points
    Hi Lloyd,

    Can you clarify that the +/-60mV range is static? For example, is Vout(10V) = 60mV and Vout(5V) = -60mV? If so, what is preventing you from solving this application with just a couple amplifiers to scale the input to the output range?

    Thanks,
    Paul
  • 0 in reply to Paul_Frost
    Expert 1710 points

    Paul,

    Sorry to be unclear. What I meant when I said "The code value written to the DAC sets what the output voltage is" is this: When the input voltage is 10VDC, I need to be able to program the DAC to generate any voltage between -60mV and +60mV. When the input voltage is +5VDC, I need to be able to program the DAC to generate any voltage between -30mV and +30mV. In between +10v and +5v input voltage, the output needs to scale, so that at +7.5V input, my possible output range is +/-45mV. The output voltage needs to be a programmable fraction of the input voltage.

    Thanks,

    Lloyd

  • 0 in reply to lslonim
    TI__Guru 53773 points

    Hi Lloyd,

    I recommend the DAC8811 - a 16-bit MDAC.  You can use two external amplifiers to scale the output to your desired range.  I have attached a TINA-TI simulation showing the behavior.  Unfortunately we do not have an MDAC model, so I created a functioning model out of discrete resistors and switches.  Basically your 5-10V input is used as the reference.  The MDAC and the first amplifier will convert the output to -5V-10V to 0V, depending on the reference value.  The second stage uses the reference to create an offset to center the output range around 0V, as well as scale the output to 60mVpp to 120mVpp (assuming full scale/zero scale DAC outputs).  I think this will accomplish your goals.  In addition, note that there will be some current demand on the reference, so you may need to buffer it.  You could also add noise filtering on the reference input if that is a requirement of your application.

    Thanks,

    Paul

    MDAC Rescale e2e.TSC

  • 0 in reply to Paul_Frost
    Expert 1710 points
    Paul,
    Thanks very much, I appreciate the help.
    Lloyd