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F2812 AD Converter questions

allegrocantabile
Prodigy 70 points

Hi, 

I have some questions regarding AD converter reference voltage pins named ADCREFP and ADCREFM.   Do any of you have answers to the following questionnaire?? 

 

Q1.  I am using internal reference for our design; which sets both ADCREFP and ADCREFM as output mode.  (ADCCTRL3.bit.EXTREF = 0).  What is the output driving capability of those two pins?  I am assuming 4mA/-4mA for output, since the datasheet mentiones all except Group2 output falls under this category. 

 

Q2.  I would like to use ADCREFP’s output voltage level as the reference to one of the external components.  According to page 65 of F2812 document(sprs174r.pdf),  connecting to external circuit to both ADCREFP or ADCREFM is discouraged.  If we connect the output of ADCREFP to an opamp based infinite buffer input, would it cause any issue to AD converter stability or operation?  If datasheet's 4mA driving capability applies to ADCREFP and ADCREFM, I feel that the driving capabilities should not be an issue. 

 

Q3.  My feeling is that opamp infinit buffer should have high-Z input that should not cause loading to the output of ADCREFP.  If this is not encouraged, could you give us the reason why this should not be done? 

 

Appreciate your help!

  • 0
    Genius 5170 points

    You are most likely correct on Q3 but I believe TI position is not to load those pins in any way and not to use those pins as any external reference.  Unless the situation is changed - those pins should be left alone :-) Haven't used 2812 for a while now so could be way off .... let's wait for comments from TI folks

  • 0 in reply to Arefeen
    Prodigy 70 points

    Hi Arefeen,

    Thanks for your comment.  That's my feeling, but I would not want to do things that would go beyond the scope of their recommendation. 

    It would be nice to know the reasoning behind it.  Like you've mentioned, I will wait for TI folks.

    Thanks again for your help!

    regards,