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TM4C123GH6PM: ADC DIFFERENTIAL MODE

uday mourya
Prodigy 10 points
Part Number: TM4C123GH6PM

Iam using a TM4C123 series Launchpad and I wanted to use the ADC of the MCU in differential Mode.

I have gone through the Datasheet of the microcontroller But I could not understand the differential mode completely. Iam new to differential ADC's so would like clear my doubts

My Input signal is a 10 Hz sinewave (+/- 1V) or (2Vp-p) . Few of my questions are :

1.The ADC is unipolar differential Input or Bipolar differential Input?
2.VIN+ can have only positive input voltage or we can send negative voltage also? Will this pin read the negative cycle of the sinewave?
3.What voltage range should be given to VIN- Pin? Can we give it a constant voltage?
4.What is Input Common Mode voltage and what relation it has with the ADC Conversion?
5.If I amplify a low amplitude signal with an amplifier , What will be the maximum Gain I should set to be witihin the range of ADC? ( For Eg. Suppose the Original Signal is (+/- 1mv). How much Gain should I set?)

  • 0
    TI__Guru 72500 points
    In both single ended mode and in differential mode all ADC inputs must be in the range of 0V to Vdda (typically 3.3V).
  • 0
    Guru 117855 points

    Follows  quick  detail - 'keyed' to your questions:   (flight awaits - this must be FAST!)

    1.  As vendor well noted - the ADC accepts only UNIPOLAR (>= 0V) inputs.   (ALL Pins!)
    2. As above - voltage (<0V) is NOT allowed!   And yes - via a proper use of 'Voltage Offset' - you can accommodate waveform's negative cycle.     Read (below) for an implementation.
    3. Providing a constant voltage to V_In(-) suggests that your 'Input Signal' is NOT Differential.    Much benefit of  such (balanced) differential signaling - will be LOST!
    4. Common Mode voltage is that which is 'impressed' uniformly - upon both Differential Input Signals.   (this usually is 'noise.')    A proper 'Differential Amplifier' will REJECT this common mode voltage - while 'passing/accepting' - the 'proper differential (balanced) voltage.'
    5. From the above - you should (now) realize that the, 'PEAK Differential Voltage' - presented to the ADC - must not exceed  'VDDA - VSSA.'    (or the device's VDD-VSS - when (no) VDDA exists  *** which appears (your) case!)    Re: your example - w/2mV Peak Signal - the gain would be 3300/2 - would it not?     And - that proves EXCESSIVE for a Single Gain Stage - perhaps even for TWO Gain Stages!    (Note that your signal will ALWAYS contain a 'noise parameter' - and  THAT NOISE  will be amplified (just) as is your desired signal!     (unless you deploy 'advanced signal techniques')

    To accommodate (near) or 'AC' waveforms - which present symmetry both 'Above & Below' a  base reference voltage - you 'Gain Compliance' with the MCU's input voltage constraint by, 'OFF-SETTING your  Amplifier to 'One-Half' of the ADC's Maximum Voltage Input.   (i.e. 1.65V)    In that manner - your Differential (+) will see a voltage of ~1.65 (above) that 'OffSet Voltage'  (~3.3V above Gnd) - while the Differential (-) will see a voltage of ~1.65 (below) that 'OffSet Voltage' (~0V.)    

    You should note that as your 'Bi-Polar' signal approaches 0V - your Differential Amplifier outputs will 'CONVERGE'  to that 'OffSet Voltage.'    (i.e. ~1.65V)    Each will then 'Move' (with symmetry) - one Above - one Below - that 'OffSet Voltage Level' - as the bi-polar input signal raises in level...

    All clear?    Now Rolling - Wheels up - Adios...