ADC input voltage

Discussion in 'General Electronics Chat' started by TrevorP, Nov 6, 2010.

  1. TrevorP

    Thread Starter Active Member

    Dec 8, 2006
    55
    0
    Hey,

    I'm looking at using a PCM1808 ADC however something is kind of confusing me with the datasheet. The Electrical Characteristics table gives a center voltage of 0.5Vcc, and the absolute max for the analog input is from -.3V to 6.5V. This makes sense as the power supply range is only 5V to 0V. What doesn't is the recommended circuit has a 1uF capacitor in series with the analog signal. That would completely remove the .5Vcc DC offset that would be required. What am I missing here?

    Datasheet:

    http://focus.ti.com/lit/ds/symlink/pcm1808.pdf

    Thanks,

    Trevor
     
  2. timrobbins

    Active Member

    Aug 29, 2009
    318
    16
    That ADC is optimised for audio signal processing - it has special features to remove very low frequency and dc components - the 1uF is the standard ac coupling technique it proposes.
     
  3. TrevorP

    Thread Starter Active Member

    Dec 8, 2006
    55
    0
    Yeah for sure. I just wanted to figure out whether or not I needed to bias my audio signal by Vcc*0.5. It would seem though that I clearly do not have to. My guess is the internal reference voltage being generated gets applied internally to the input. Which would satisfy both their recommended circuit and their absolute max. ratings.
     
  4. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
    Don't guess there is an internal reference voltage, refer to the datasheet for the requirements.

    Most provide a bias, some do not. The peak voltages need to be considered, so a 3V negative going peak doesn't go below -0.3V, etc. Ensure a signal doesn't have that much swing/amplitude before entering the ADC.
     
Loading...