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TM4C123GE6PM: USB connection adds DC offset to ADC in Tiva part

Robbie Valentine
Expert 2975 points
Part Number: TM4C123GE6PM


Hello,

I have a product that uses the ADC in the TM4C123GE6PMI part and communicates to a PC via USB.  I'm having a problem where the USB connector adds a DC offset to my PCB/circuit when I plug USB into the tiva part.  Is there a known way to mitigate this?  It creates a problem when sampling data coming into the ADC.  This is exacerbated by gains in the analog front end that gain up my incoming signal.  The offset added by USB is gained up as well which puts half of my signal out out side of sampling range.  I can't seem to find much info regarding this.  Any info/advice is greatly appreciated!

  • 0
    TI__Guru**** 191906 points

    Hi Robbie,
    How are the GNDA for the ADC and the GND for the USB connector routed and grounded on your board? Do they have a common ground?

    Below is how the USB connector in the Launchpad is grounded for its shield.

  • 0
    Guru 117855 points

    Robbie Valentine said:
    ...the USB connector adds a DC offset to my PCB/circuit when I plug USB into the tiva part. 

    This writing suggests that you have (another) means of powering your board - is that correct?    (i.e. you do NOT note this offset when the board is "free" from PC attachment)

    Unstated is the extent of this unwanted Offset - that detail may aid diagnostics.

    Would it not prove useful to measure all key/critical power supply voltages (both) before & post the "plug-in" of USB.   All critical MCU power voltages should be measured as well.

    Should that MCU employ a separate pin for VDDA - it is usual to provide the cleanest, most filtered voltage possible.   (i.e. NOT that voltage which feeds MCU's VDD pins)

    Your gain stage (amps) should be "R2R" (rail to rail) to (usually) best null such offsets - is that the case?    Does the Powering or reference voltages @ these gain stages vary as USB connects?

    Another possibility - (we hope not) - the USB cable ties your MCU board's ground to the PC's ground - which may present a challenge.

    As vendor's Charles notes - board layout - robust power traces - and often "Single Point" Ground usually provide best results...

  • 0 in reply to cb1_mobile
    Expert 2975 points
    Guys,

    Thank you for the responses. Let me do some more research this week and post up some more info. I really should have provided the details and data cb1 mentioned. I'll be back with that information.

    Thanks!