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TMS320F28P650DK: Limitations of ADCs in differential mode

Akira Sugimoto
Prodigy 90 points

Part Number: TMS320F28P650DK

Tool/software:

Hello Team,

I am designing the voltage monitoring circuit which outputs to a CPU ADC in differential mode. 

The Figure in the data sheet includes a statement of "Vin Common Mode limitation" it can be interpreted that "Vin common mode" must be less than VREFHI/2 ± 50mV. My understanding is that the conversion range (16-bit Differential) is from VREFLO to VREFHI, hence ADCINxP and ADCINxN both can be from VREFLO to VREFHI and the actual input voltage range (ADCINxP - ADCINxN) is about VREFHI - VREFLO. What is the "Vin Common Mode" in detail, how does it impact the input? (I found about this topic in related threads, but I could not fully clear up my question.)

Thanks.

  • 0
    TI__Intellectual 2270 points

    Hi,

    Please find the below explanation for the Differential ADC drive:

     

    This is a snippet form the F28379D launchpad, where a diff amp is used to provide the input to the ADC where Vcm voltage is kept at VREF/2 +/-50mV. 

    One example of providing DC input to the ADC: 

    Input need to be measured is 0.5V and VREFHI -VREFLO==> 2.5V

    +ve Input (ADCINA0) --> 0.5+2.5/2 = 1.75V

    -ve Input (ADCINA1)--> 2.5/2- 0.5V  = 0.75V

     

    Thanks,

    Nilesh 

  • 0 in reply to Nilesh Khanna
    Prodigy 90 points

    Hi Nilesh,

    Thank you for your reply.

    Would it be accurate to understand it that the INPUT voltage is adjusted with an appropriate voltage so that the input common mode voltage is kept at VREF/2 +/-50mV? You gave the example (0.5VDC input), but does differential input come out to 1V? Is each (0.5V positive/0.5V negative) different situation?

    The snippet image link is broken, could you please insert the image again?

    And I would like clear information about voltage range of each individual input and measurable range in differential mode when VREFHI=3.3V.

    Thanks,

    Akira

  • 0 in reply to Akira Sugimoto
    TI__Intellectual 2270 points

    Hi Akira,

    Akira Sugimoto said:
    Would it be accurate to understand it that the INPUT voltage is adjusted with an appropriate voltage so that the input common mode voltage is kept at VREF/2 +/-50mV?

    Yes that is correct. 

    Akira Sugimoto said:
    but does differential input come out to 1V? Is each (0.5V positive/0.5V negative) different situation?

    yeah you are right i made a typo there it is for 1V. 

    Re-inserted the image:

    Input range is limited to VREFHI --> your case 0to3.3V.

    BR,

    Nilesh  

  • 0 in reply to Nilesh Khanna
    Prodigy 90 points

    Hi Nilesh,

    Thank you very much for the detailed explanation. I understand.

    Best Regards,

    Akira