Where & how to block DC?

Discussion in 'General Electronics Chat' started by atferrari, Feb 18, 2012.

  1. atferrari

    Thread Starter AAC Fanatic!

    Jan 6, 2004
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    My design of a DDS finally works OK but I want AC at the output. Block diagram attached.

    My questions:

    What is the correct place to get rid of the DC level?

    A cap alone, is it the right way? If so, thinking of an output of 5Hz to 16 KHz what would be the right value?

    Right now I have available non-polarized caps of 2uF and little space to parallel some.

    What if I resort to an electrolytic or tantalum? How to know where the "+" terminal goes to?

    Gracias for any help.
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    Remember that in any circuit there will be some R. So you have to take this into account and calculate the roll-off frequency:

     f = \frac{1}{2\pi RC}

    It is best to use non-polarized capacitors. If all you have are low value aluminum electrolytics, these are ok to use. At low voltages I would not worry too much about polarity but you can use a DC voltmeter across the cap in circuit and figure out which end is more positive.
     
  3. atferrari

    Thread Starter AAC Fanatic!

    Jan 6, 2004
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    Thanks for replying.

    The pot already at point (C) is 10K. I think I could use it with a cap in series between the buffer and the amplitude control stage. Am I right?

    What is the roll off frequency I have to look for? I am at lost here.
     
  4. MrChips

    Moderator

    Oct 2, 2009
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    If you had a total series resistance of 10K in series with 2uF that would give you a roll off at about 8Hz.
     
  5. atferrari

    Thread Starter AAC Fanatic!

    Jan 6, 2004
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    After revising my options I came to the attached circuit.

    1uF and 100K seem to work OK.

    In way of recalculating the filter (with higher fc) to have the last KHz unattenuated.

    Gracias for replying.
     
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