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TM4C1230E6PM: ADC Resolution in differential mode

SY Yoon
Genius 3380 points
Part Number: TM4C1230E6PM

Hi, 

I have a question about ADC differential sampling.

My customer connected VIN+ to the analog input and VIN- to GND on board.

The board has an input voltage range from 0V to 3.3V.

When the input is 0V in differential mode, the conversion result is approximatively 2048

When the input is 3.3V in differential mode, the conversion result is approximatively 4095.

According to the ADC conversion result, It seems resolution of positive voltage range is 2048.

Is there a way to make the resolution of the positive voltage to 4096 in differential mode?

My customer use only positive voltage range. 

  • 0
    TI__Guru**** 176826 points

    Hi,

      The behavior is expected. As you can see from the datasheet description, when VIND > 0 then the conversion range is 0x800-0xFFF. 

  • 0 in reply to Charles Tsai
    Genius 3380 points

    Hi, 

    I checked datasheet. 

    My customer wants to use the range 0x000 ~ 0xFFF when VIND > 0 in differential mode.

    If the conversion result is 0x000 ~ 0xFFF when VIND > 0, the resolution of the VIND > 0 input is considered as 2048.

  • 0 in reply to SY Yoon
    TI__Guru**** 176826 points

    Hi,

      The reason is that in differential mode the input range is extended in the negative direction such that differential voltages of -Vref and Vref gives the minimum and maximum input values, respectively. The corresponding digital values for the valid analog input range [-Vref,Vref] will be signed integers in the range [-2n-1,2n-1-1], where n is the resolution of the ADC in bits. For example,  A 12-bit differential ADC with signed output will commonly give digital values in the range [-2048,2047]. The resolution is still 12 bit. It is that in your usage the Vin+ is always greater than Vin- so you will never be in the the range between [-Vref, 0].