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TMS320F280041: ADC register value read abnormally

Cickey Xu
Intellectual 736 points
Part Number: TMS320F280041


dear colleagues,

My customer use register TMS320F280041PZS to design product.

when they connect 3.3v voltage to VREFHIA/VREFHIC and certain input channel(such as ADC-A Input 0),if  they select internal reference voltage,the ADC register value is 2045, and if they select external reference voltage,the ADC register value is 4095,

but when disconnected  3.3v voltage to VREFHIA/VREFHIC,if  they select internal reference voltage,the ADC register value is 4095.

can you tell me why?

thank for your support.

Cickey Xu.

  • 0
    TI__Guru 57255 points

    Hi Cickey,

    The ADC in the F28004x device is 12-bit.  So the full-scale range of the ADC register's value is 2^12-1 (4095)

    1) 3.3V to VREFHIA, external reference mode, and 3.3V to an ADC input => ADC input is equal to the external reference voltage, so register value should be ~4095.
    2) internal reference mode and 3.3V to an ADC input => with internal reference, the full scale range is 0 to 3.3V for 0 to 4095, so register value should be ~4095.
    3) 3.3V to VREFHIA, internal reference mode, and 3.3V to an ADC input => this is not allowed and will likely damage the device. 


    Thank you,
    Brett

  • 0 in reply to Brett Larimore
    Intellectual 736 points

    dear Brett,

    Thank for your support, i got your meaning.

    I feel uncertainly how should compute ADC value, if consult datasheet below equation, if i use internal reference,there are two reference voltage,1.65v and 2.5v

    ,if configure default value, the chip select 1.65v reference; otherwise, the chip will select 2.5v reference.

    I  want to know , if use internal reference voltage,in two reference voltage mode,

    1.what is the input channel voltage range?

    2.if i connect 2.7v voltage to certain input channel, what is the ADC register value(need Separatly calculation  1.65v voltage reference and  2.5v  voltage reference)?

    thanks again.

  • 0 in reply to Cickey Xu
    TI__Guru 57255 points

    Hi Cickey,

    If using the 1.65V internal reference mode, the full scale range of ADC conversion will be between 0-3.3V.  Internally, the ADC inputs are attenuated in this mode.

    If using the 2.5V internal reference mode, the full scale range of ADC conversion will be between 0-2.5V.

    Therefore, if 2.7V is input to an ADC channel:
    - with 1.65V reference, the ADC result will be approximately (2.7/3.3)*4095.  ~3351
    - with 2.5V reference, the ADC input will be greater than the reference voltage.  The result will be therefore be ~4095.

    Hopefully this helps!


    Thank you,
    Brett

  • 0 in reply to Brett Larimore
    Intellectual 736 points
    Hi,Brett
    thank for your great support,i got it.