• State Resolved
  • Locked Locked
  • Replies 5 replies
  • Answers 2 answers
  • Subscribers 46 subscribers
  • Views 262 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

AMC7834: Bipolar DACs - negative Voltage (-5V) and currente consumption at AVss

Marks Greulich
Intellectual 530 points
Part Number: AMC7834

Hi guys,

I`m using the AMC7834 and happy to have him.

But what I dont understand how I should scale the power supply at the AVss side which is -5V at my case. So that is the lowest voltage what is needed to set via the bipolar DAC.

So what is the maximum current what one of that DAC could drive? 10mA?

And where does the current come from - directly from the AVss supply? So that needs also to deliver 10mA? Or does the current come from e.g. AVdd?

Question behind - I have several of that AMCs on my PCB to drive several devices with it but I have problems to find an LDO/DCDC shifter that generates the -5V and such a high current at the -5V side (e.g. 120mA).

Thanks in advance,

Markus

  • 0
    Intellectual 530 points

    And one next issue:

    There is written in the datasheet that the Bipolar DACs could drive min. +/-10mA. Does that mean for each of that 4 bipolar DACs or for all of the 4 or for each group (DAC1/2 and DAC3/4)?

    Thx,

    Markus

  • 0 in reply to Marks Greulich
    TI__Guru 52683 points

    Hi Markus,

    1. If you want to be able to sink current while the output is -5V, then you need to have some supply headroom.  If you look at figure 24 in the AMC7834 datasheet, you will see that there output shifts about 300mV when sinking 15mA.  This is because the DAC has no headroom (VSS=0V) in this case.  If you want current sink/source capability, then you should have a supply lower than -5V: I recommend -5.5V.

    2. We specify a minimum of ±10mA for sink and source current when the DAC is operating in the linear region.  Really what we are saying is that if you sink or source 10mA with adequate headroom (at least 300mV), then the output should not shift more than 25mV.  That is for each channel.  Now your question was how much current can it source/sink.  Well, that would be more than 10mA per channel.  When the current load is beyond 10mA, the DAC will still try to maintain the output voltage until it reaches a short circuit condition.  At this point, the output will slew to the supply rail (see figure 22).  We specify that it is typically 45mA.  So the worst-case current would be all 8 DACs in short circuit condition, or about 45mA×8.

    3. If the DAC is sourcing current, the AVCC would need to provide it.  If it was sinking current, AVSS would need to sink it.  

    If you find that the AVSS is sinking more current than expected, then you should look at the loads that you have on the DAC.  Feel free to share the schematic if you would like.

    Thanks,

    Paul

  • 0 in reply to Paul_Frost
    Intellectual 530 points

    Hi Paul,

    thanks a lot for that detailed answer!

    To be 100% sure that I understood it correctly - one "channel" means one out of the DACs? I`m only using the bipolar ones and there are 4 DACs. So each could min. supply 10mA, so 40mA for 4x10mA.

    I`m asking that cause the datasheet Groups sometimes the bipolar ones DAC1+2 and DAC3+4 (Settings, voltage range...) and therefore I`m afraid that they also share the max. current together.

    Thanks and best regards,
    Markus

  • 0 in reply to Marks Greulich
    TI__Guru 52683 points

    That spec should be considered "per channel".  The worst-case current consumption from the DAC output be if they are in short circuit condition, where you have a typical max of 45mA × 4 channels = 180mA.

    Thanks,

    Paul

  • 0 in reply to Paul_Frost
    Intellectual 530 points

    Thanks Paul, answered!