volt dip on piccolo controlstick 28027

Hi Weimin,

you have to be more specific where exactly did you observe this voltage dip? What else is on that line?

Regards, Mitja

Frank,the interval is about 18us,not 18ms.

I think it can't be power line noise.

Hi,Mitja,

I add a stabilized volt to ADC-A2 of piccolo demo board(28027), then connect probe to ADC-A2,when I run example project, I got this.

To observe ripple better,I switch oscilliscope to AC mode, pk-pk ripple value is about 180mV

Can you post a schematic of everything in between the voltage you are measuring and the ADC-A2 pin?

weimin ren said:

Frank,the interval is about 18us,not 18ms.

I think it can't be power line noise.

Do you have a ~55kHz noise source nearby? Some PWM for example?

And finally, is this only affecting the A2 pin? Can it be observed when trying a different input?

Tim

My guess would be that you are sampling at 55kHz, and what you see is transent when ADC samples the channel. You should either drive the line with low impedance circuit or add a tank capacitor (you might also add resistor in series in order to suppress oscillations of opamp-if you have opamp in front)

Mitja 

Weimin,

All of what the others above are potentially the issue.  I have one other thought that may be also be worth experimenting.

If the issue is mainly occurring on ADC-A2 or A4, I would suggest trying to disable the comparator clocks and see what happens.

If this fixes your noise issue, then your issue is probably with the hysteresis functionality of the comparator.  If you need to use this comparator (for overvoltage trip, etc), you'll need to disable the hysteresis feature (which is unfortunately currently undocumented) and then you can use the comparator.  See the following thread: 
http://e2e.ti.com/support/microcontrollers/tms320c2000_32-bit_real-time_mcus/f/171/p/110662/391516.aspx#391516


Thank you,
Brett