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TMS320F280021: F28002X Internal Reference Question

Dominik Lorenz25
Prodigy 140 points
Part Number: TMS320F280021

Hello Ti,

I'm designing a Board featuring a F280021. In the Technical Reference Manual on page 1495, in Point "15.2.3.2 Internal Reference Mode" it is stated that the internal Reference Voltage can be set to 1,65V. Furthermore it is stated that:

"When a 1.65V internal reference voltage is selected, the effective ADC conversion input range will be VREFLO to 3.3V"

Can you elaborate on that (Especially where the factor 2 comes into play)? When I read figure 15-1 (which is the same as 7-32 in the Datasheet) correctly, the Voltage is directly applied to VREFHI. Furthermore, as stated in Table 15-2, an Input Voltage greater that VREFHI results in a full scale reading. Therefore I would conclude that the conversion input range would be VREFLO to 1.65V. The Datasheet states in point 7.12.1.2.1 that VREFHI - VREFLO has to be at least 2.4V and that VREFLO has to be tied to VSSA, therefore 0V. A reference voltage of 1.65V woud not be consistent with that specification.

Obviously, I am missing or misunderstanding something. I would be grateful if you could explain.

Thank you very much,

Dominik

  • +1
    TI__Guru 60865 points

    Hi Dominik,

    In internal reference 1.65V (3.3V range) mode the internal reference voltage is effectively doubled before being used in the ADC (or the ADC input is divided down by a factor of 2 before being converted  - whichever way you want to think about it). 

    Probably the DS line item that specifies "VREFHI - VREFLO has to be at least 2.4V" should specify that it only applies to external reference mode. 

    Either way, using 1.65V mode is functionally equivalent to forcing in 3.3V into VREFHI in external reference mode for the ADC: a 3.3V input will produce a full-scale reading and a 1.65V input will produce a half-scale reading from the ADC. 

    Agree that there is some ambiguity in the documentation; I'll submit a ticket to have things made a little clearer.   

  • 0 in reply to Devin Cottier
    Prodigy 140 points

    Thank you very much!