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Reading ADC0804

Moacir Silveira Junior
Prodigy 50 points

Hi all,

I making a project wich uses a ADC0804 to read one resistive touch. Is working fine, but have one problema.

The resistive touch only suply in analog input 0v to 2.5v, but I reading only 0 to 80H (0 to 128), not 0 to 255, and this was complicating my conversion to prosition.

I haver leave the pin vref/2 NC. How i inform to ADC0804 wich is to last voltage is 2.5v and not 5v ?

Ty a lot

moacir jr.

  • 0
    TI__Guru**** 180931 points

    Hi Maocir,

    Can you give us any more detail on how you have your ADC0804 hardware configured?  Perhaps a schematic or some timing (O-Scope) captures?

  • 0
    TI__Expert 4340 points

    If I understand correctly, your sensor outputs a voltage from 0V to 2.5V and you would like that span to map to the full 255 codes of the ADC0804 but right now it only maps to 128 codes.

    The reference configuration on the ADC0804 is a little tricky. If the VREF/2 pin is left NC, then the VREF/2 voltage is internally set to VCC/2 (half the supply voltage). If you are using a 5V supply, the VREF/2 voltage would be 2.5V.

    Now section 2.4.1 of the datasheet indicates that "The internal gain to the VREF/2 input is 2, making the full-scale differential input voltage twice the voltage at pin 9." So, if you use a 5V supply, then the full scale range of the input is 5V.

    To set the full scale range of the ADC to 2.5V, you must force a 1.25V reference on pin 9 from a low impedance source.

    Regards, Josh

  • 0
    Prodigy 50 points

    Very Thanks,

    Now, the conversion is a success. Ty a lot, I was reading the vref/2 in datasheet, but I dont understand.

    Now I have other problem, but I will post in other question.

    This question is SOLVED.

    Moacir Jr,

  • 0 in reply to Tom Hendrick
    Prodigy 20 points

    HI All,

    Could somebody help me? Where can I find the Spice model for the ADC0804 converter? http://www.ti.com/product/adc0804-n

    I can't find it in the official model list: http://www.ti.com/adc/docs/midlevel.tsp?contentId=55102

    Thanks in Advance! Anthony