K28 VDDA pin for adc conversion power down

[jasonlarson]

If I want to save power and only use the adc's in the K28 very little meaning once ever couple of days can I power down this interface by floating it or tying it to ground to save the 215ua of current the data sheet says is the minimum current consumption? Reading through the data sheet it looks like VDDA is only used for the ADC logic correct? I did get info on another post saying I can float the vreh pin even though the data sheet says it should be tied to a voltage so I'm hopeing the same thing will apply for the VDDA pin.

[jasonlarson]

John,

 Thanks for the reply. I ask the question about the Vreh and got this answer  https://community.nxp.com/message/419427?commentID=419427#comment-419427. Are you saying the answer was wrong? Based on that answer is why I asked the question about the VDDA pin. Beside the ADC and DAC which modules use the VDDA supply?

[john_suchyta]

Hi Jason,

The recommendation is to tie VREFH to VDD potential if the ADC is not used. The previous answer is correct in its context since the ADC is used and an external reference is used. even though it can float. VREFH is not a supply pin.

VDDA is a power domain, so it should not float or be grounded. The primary risk is latchup, but there are level-shifters that interface between the VDDA and VDD domains that need to be powered. If this is the K28F device, the ADC, DACs, and VREF_OUT are powered by VDDA.

Regards,

John

[john_suchyta]

Hi Jason,

VDDA always needs to be biased. VREFH always needs to be biased. These pins cannot float. The VDDA supply is used for all analog modules on board, not just ADC. Not connecting it to the VDD potential causes a risk of latchup. The only analog pins that can be floated if they are not used are the ADC inputs, VREF_OUT, DAC_OUT, and other special analog I/O.

We do not recommend a separate supply for VDDA since it needs to be at the same potential as VDD for the level-shifters to work properly between digital and analog domains. VREFH can have a separate external reference if desired.

Regards,

John Suchyta