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MSP430 ADC Help me please

Moon Kim
Prodigy 40 points

Hi. I'm a noob with msp430 and I couldn't figure this out with datasheet. This might be b simple question but I'm posting here anyway.

Im trying to convert an analog signal which changes from 1.65v to 3.3v to digital representation. How to I set adc's low voltage limit to 1.65 and upper limit to 3.3v when I can get those voltages from external sources?

cheers

  • 0
    Mastermind 7395 points

    On the MSP430 (at least for the models that I know), you can't. The ADC will expect for signals from 0 to 3.3V (if you connect 3.3V to AVcc). So if your ADC has 12 bits, you are effectively "deleting" one bit from it, since the signals vary from 1.65 to 3.3V. You get an equivalent accuracy between 10 and 11 bits.

    It that is unacceptable, you can put an external summing amplifier that will add an offset and gain to convert the (1.64, 3.3V) range to (0, 3.3V).

  • 0
    Guru 227245 points

    Moon Kim said:
    How to I set adc's low voltage limit to 1.65 and upper limit to 3.3v when I can get those voltages from external sources

    Some MSPs allow external referecnes to be used. THen there is an Veref+ adn Veref- input where you can apply 1.65 and 3.3V. This willr eturn a value 0 0 for 1.65V and below and a value of vmax (0xff, 0x3ff, 0xfff, depending on ADC resolution) for 3.3V or above. It implies that VCC is at least 3.3V,

    Alternatively, you can just sample the voltage as-is, accepting that your values will always be above 50%. Of course this results in a smaller resolution (you lose 1 bit, as you are using only half of the input range)

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