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TMS320F280049: Connecting VrefHI to VDDA

Brian Wang0
Expert 5360 points
Part Number: TMS320F280049


Hi team,

One of my customer is asking whether they can connect the VrefHI to VDDA when external referrence is used.

They reason why they want to do this is their concern about the temperature drift about C2000 internal reference. And they are quite satisfy the performance of their VDDA. Similar methods have been used in their previous products.

Thus, I have the following 2 questions:

1. I noticed on our datasheet, the typical VREFHI is 3V while the MAX VREFHI is VDDA. Will connecting VREFHI to VDDA has any bad influence on the ADC sampling?

2. What is the temperature drift parameter of our internal reference? I did not notice this parameter in our datasheet.

  • 0
    TI__Guru 60865 points

    Hi Brian,

    The temperature drift of the internal reference is essentially the delta in the gain error specification between internal reference mode vs external reference mode.

    VDDA can be connected to VREFHI; the ADC will convert correctly.  However, the effective PSRR will be 0dB and the ADC results are likely to be extremely noisy.  

  • 0 in reply to Devin Cottier
    TI__Expert 5360 points

    Hi Devin,

    Sorry that I did not follow you on the description.

    1. According to my understanding, the temperature drift parameter should have a unit like V/℃. However, the gain error shown in our parameter is xx LSB which is not related to temperature. How to understhand gain error and what is the relationship with temperature drift.

    2. As for the PSRR you mentioned, it is usually used to measure the capability of stablizing out voltage of power supplier. Thus I am assuming the PSRR here is used to measure the ripple on reference voltage caused by VDDA. Is that correct? If an additional DC/DC or LDO is used to provide another Vref for ADC, the PSRR performance will be better, is that correct?

    Regards,

    Brian

  • 0 in reply to Brian Wang0
    TI__Guru 60865 points

    Hi Brian,

    (1) All the specifications are across all conditions unless otherwise specified.  In the case of gain error in internal reference, the specification already includes temperature drift of the internal reference.  If you want to estimate what the drift was that contributed to that specification,  you can subtract the external reference gain error specification (because it assumes an ideal external reference) and then divide by the temperature range.  e.g. (45LSBs - 5LSBs)/(0.5*(125--40)) = about 0.5LSBs/deg. C.  

    (2)Basically if you connect the power supply to the VREFHI, you will see any power supply noise directly on the ADC output (because you'll see noise from any noise on the VREFHI input on the output).  It would be better to build a proper voltage reference circuit than to build another power supply and use it as a reference voltage.