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TIDA-010933: Why this Gain of AMC3330 and AMC1311 in the design

Naveen Gupta
Prodigy 130 points
Part Number: TIDA-010933
Other Parts Discussed in Thread: AMC3330, AMC1311,

Tool/software:

Hello experts,
I am currently designing a voltage sense system for my project, both AC and DC.
So, when I was studying the design for TIDA-010922, I saw that these ICs (AMC1311 and AMC3330) are utilized for voltage sensing, which is rather straightforward to use and can be  connected to ADC.(According to sbaa229b - Interfacing a Differential-Output (Isolated) Amp to a Single-Ended Input ADC),

But the gain utilized for the DC-DC AMC1311 is 0.15, whereas on the AC side it is 0.75.
Why is that?So on the AC side sensing, 1.65V (half of 3.3V, assuming that is the voltage of the ADC) is used as Vcm or Vref.To convert a +-2V differential signal into a 0-3.3V single ended signal? Correct?But when I enter the values into the formula (given in SBAA229b), the outcome ranges from 3.18V to 0.1125V.


Am I missing something? Or did I misunderstand something?
Similar material for AMC1311?

Sorry, i am still new to this stuff.

Please let me know what i am missing.

I am attaching the SS of differentail to single ended signal stage.

Thanks and Regards,
Naveen

  • 0
    TI__Expert 3270 points

    Hi,

    if you consider the example of a AMC1311 the gain will be calculated as follows:

    From HV to LV the gain will be calculated based on the voltage divider you apply there.

    From the LV in the HV region to the LV on the isolated side you will have a gain equal to 1.

    Since it is not a sigle ended signals, you will need to convert the signals into single ended by using for example the schematic you have placed there. the gain of that stage is coming from the feedback resistance and the input resistance. Rf/Ri.

    Thanks

    BR

    Riccardo

  • 0 in reply to Riccardo Ruffo
    Prodigy 130 points

    Hi Riccardo,
    Thanks for your insight.
    Ok i did the calculation how you explained please correct me if I am wrong anywhere.

    At AC HV the voltage range to be measured is +-401V therefore resistor divider is selected as 2M,10K,2M which makes sure that +-1V drop can be seen at 10K resistor all the time which is in the limits of AMC3330.

    After that device have a fixed gain of 2 hence the differential output is +-2

    Now swing at each pin +-2/2=+-1 around 1.44V(Vcm) [Referring SBAA229b]

    So at max input voltage (ie:+1)voltage at each pin can be calculated as: 

    VoutP=Vcm+1(1 comes from the swing value calculated earlier +-2/2=+-1)
    VoutN=Vcm-1

     so the output VoutP-VoutN at each input will be something like this 




    Therfore finally the ADC calculation will be Vadc=(VoutP-VoutN)*(Rf/Ri)+Vref

    and in TIDA-010933 they have taken Rf/Ri as 0.75
    which results value for ADC as 0.15V-3.15V

    but if i i want +-2V to be +-1.65V(for 3.3V ADC)
    best gain choice will be 1.65/2=0.825

    which will give me value between 0-3.3V

    So why is the gain 0.75 selected????


    I hope i didnt made any mistake.

    Thanks and Regards,
    Naveen

  • +1 in reply to Naveen Gupta
    TI__Expert 3270 points

    In general, from theorethical point of view you can also place a 0.82 gain..

    i have keep 0.75 to have some marging. I do not recommend to go perfecly up to the 3.3V rail of the ADC if you would like to measure properly the voltage.

  • 0 in reply to Riccardo Ruffo
    Prodigy 130 points

    Thanks alot for the information I think i understand this perfecly to go ahead and design my system for both AC and DC.