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Sampling Audio signal (Music) with the MSP430 family

Keroloa Abadeer
Prodigy 20 points

Hi,

I need to use the MSP430 on chip ADC to sample a music input signal. The issue I am having is that the analog music signal has ground at 0V, and it swings from negative values to positive values (like a simple sine wave), and the ADC doesn't take negative values. How do i "condition" the input signal (raise the ground to a mid point value like 1.5V) before it enters the ADC chip, so that the entire wave becomes positive (above zero), and therefore be able to preform the A/D conversion correctly? because otherwise the ADC will simply ignore the negative portion of the signal, and only sample the positive half of the signal (values that are above zero)!

Is there a recommended external circuit to do that?

Thank you in advance.

Kerolos

  • 0
    Genius 3155 points

    You could use a non-inverting op-amp level shifter like this

    http://www.daycounter.com/Circuits/OpAmp-Level-Shifter/OpAmp-Level-Shifter.phtml

    Regards,

    Peppe

  • 0 in reply to Piergiuseppe Tundo
    Prodigy 20 points

    Peppe,

    thank you so much for your reply. That is exactly what I need. I spend hours looking for a solution like this, but no luck (until now).

    Again, thanks a lot.

    Kerolos

  • 0
    Guru 227245 points

    There are several possible ways to solve this.

    1. there are some MSPs which have an ADC that allows +-0.6V symmetrical input. These are the devices designed for energy metering.
    2. you can connect the GND level of the music signal to 0.75V or 1.25V (virtual ground). This allows the ADC to sample 0.75V+-0.75V or 1.25V+-1.25V signal (for 1.5V and 2.5V reference)
    3. you can add an offset voltage to the signal using OpAmps.
    4. you can sample the positive half-waves only and mirror them to the zeroed portion of the sampled signal, where ht enegative half would have been. audio waves don't change so much that assuming the negative half-wave being a mirror of the positive is such a big difference. At least as long as you filter out really low high-energy waves. It adds, however, some distortion, depending on your signal. Interestingly it works best with sine waves (which are usually the most sensible signals to distortion)

     

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