LAUNCHXL-F28379D: ADC External power supply mode

Hi Mohamed,

U11, U13, and U19, which generate and buffer the VREFHI voltage, are supplied by +5V, so you'll also need to supply +5V externally in addition to the +3.3V. 

but if you see in the link below they said that the board will never be supplied by 5v the 3.3v will be step up for that purpose

https://e2e.ti.com/support/microcontrollers/c2000/f/171/t/670593

if they are wrong please give the configuration how to supply both 3.3v and 5v in the same time 

Hi Mohamed,

The thread say that to enable the step up, JP6 should be populated.  Is JP6 populated? Do you read 5V on the 5V rail?  

yes

Hi Mohamed,

Can you clarify the following:

"the adc always read 3.3v"

Because the VREFHI generated by the board is 3.0V, if the ADC outputs the max code (4095 in 12-bit mode) this would correspond to 3.0V.  It therefore shouldn't be possible for the ADC to produce an output corresponding to 3.3V. 

"the signal get cut if input it in the ADC"

Which pin are you applying the input to, what is the signal source, and what do you mean by "cut"?  Do you use a DMM to read back the actual input voltage? If so, what voltage is present and what is expected? Is the input being pulled high or low?  

hi i was able to make the ADC SHOWS the sine wave by supplying both 3.3v and 5v in the same time but the sine wave is very distorted not as pure as if supply the board from the pc do you have any idea how to solve this?

and by the way i am using the differential ADC option not the single ended 

this how my 1hz sine input look like

Hi Mohamed,

Is the signal range actually about +1.5V to -1.5V?  The electrical range of the input (for both positive and negative halves of the differential signal) needs to be within 0 to VREFHI.

Also, is this the positive or negative half of the differential input?  What does the complementary half look like?  Are you sure the common mode is VREFHI/2?

I think "Figure 5-31 Differential Signaling Mode" in the datasheet is helpful for understanding what this looks like:

This 1hz 1v sine  from a function generatorwhich has been given to the same inputs pin in the same board while the power supply is taken  from the usb and showed pure sine but the pic shown above is when i used external power supply

Hi Mohamed,

But are the two halves of the differential signal strictly within 0V to 3.0V and is the negative differential input the compliment of the positive input around VRFEFHI/2 (the common mode voltage) and not something else (for example, connecting the negative input to ground will give incorrect/unpredictable results)?  If not, the results that you get will be incorrect and may vary unpredictably.

Yes its a sine wave 1vpp  and its symmetrical and the proof of that that its work fine the same board with same   input signal same wire but the supply its  not externally but from the usb a d it show pure sine , what's in the image only happened if i powered the board externally

Hi Mohammed,

I'm still not clear on exactly what the issue is in the scope capture you shared. 

I think it would be better to take a step back.  For both the working and non-working case, can you share?:

• Scope capture of both inputs (ADCINp and ADCINn) indicating the absolute voltage range
• Raw digital waveform produced by the ADC (0 to 65535 scale)
• DMM measure of 3.3V and 5V rails
• Listing of which jumpers/connections of JP1, JP2, JP3, JP6, USB,  External 3.3V, and External 5.0V are active