TMS320F28377D-EP: The VREFHI constraints on TMS320F28377D-EP

Nomo,

Yes, VREFHI can be tied to VDDA.  There are some architectural features that contribute to the usage quirks that you are asking about.

First is that the ADC uses a SAR design so it is sensitive to noise on the VREFHI signal.  In order for the ADC to meet the performance numbers in the datasheet, the VREFHI needs to be strongly filtered -- this is why there are minimum requirements for localized VREFHI bypass capacitance and the recommendation to buffer the source reference signal.

Second is that there are ESD protection diodes on the device pins that will shunt excess energy to the supply pins. This includes a shunt path between VREFHI and VDDA. Like any diode, nonlinear charge conduction on the ESD diodes can start to creep in at relative small voltage potentials.

The combination of these two features makes it tricky to tie VREFHI to VDDA. In order to achieve good ADC performance, VREFHI would need sufficient filtering in order to reject VDDA noise. However, such a filter can lead to small voltage differences between VDDA with ripple vs VREFHI with more stability. Under such scenarios, the ESD diode can inject VDDA noise into VREFHI, which works against the desired filtering.

The blanket recommendation of using VREFHI = 3-V TYP was provided to avoid these design complexities in generic systems. However, the original compatibility of VREFHI = VDDA was retained so that more sophisticated system designers can make their own tradeoff decisions.

-Tommy