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Differential Output

Discussion in 'Electronic Design' started by Apparatus, Dec 12, 2004.

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  1. Apparatus

    Apparatus Guest


    I am using a Texas Instruments TLV320AIC1106 PCM Codec for some audio
    circuitry. This codec is capable of driving an 8 to 32 ohm load
    speaker. The suggested connection topology for this speaker is
    differential output according to the datasheet, but a schematic is not
    included with this recommendation.

    I was planning to simply connect one terminal of the speaker to EAROUT+
    and the other to EAROUT-, but recently it was suggested to me that in
    addition I couple each terminal to ground with a 1kohm resistor. What
    are the advantages of this?

  2. Robert Baer

    Robert Baer Guest

  3. Robert Baer wrote...
    Correct. The purpose of a differential speaker output (often called
    bridge mode output) is to eliminate the need for electrolytic output
    capacitors when using a single-supply power source, and to double the
    available speaker voltage, thereby quadrupling the maximum power
    obtainable from the power source, which is often a small battery.
    A bridge output is fully differential, and has no knowledge of, nor
    need to interact with ground. Very small speaker amplifier ICs can
    be made using class-D (switchmode) H-bridge output stages, which
    dissipate little heat while driving the speaker. But it is necessary
    to incorporate four power MOSFETs into the IC for the speaker current.
  4. I read in that Winfield Hill <[email protected]_rowland-
    Maybe the resistors are advisable if the loudspeaker can be
    disconnected. Not to provide a load on the amplifier but to prevent
    possible excessive centre-point offset.
  5. Apparatus

    Apparatus Guest

    Thank you all for your informative answers. The speaker is
    disconnectable (headphones via phonojack), but the initial center point
    offset should only induce a click or pop sound in the headphones. Since
    this is not a problem, I'll leave out the resistors.

  6. Apparatus wrote...
    I don't understand your comment about center-point offset.
    In these balanced amplifiers, both outputs are at the same
    voltage with no signal, so the difference voltage is zero,
    and plugging in a headphone should not make a click. Note,
    the outer ring of the headphone jack shouldn't be grounded!
  7. I read in that Apparatus
    Are you sure? There are two aspects:

    1. The offset may damage the chip over time;

    2. The click may exceed the 'acoustic shock' limits. People tend to put
    the headphones on and then plug them in. Since many headphones produce
    100 dB SPL for 1 mW or less input, the possibility of getting more than
    126 dB SPL (the ITU recommended limit) is significant. I have
    experienced a 126 dB click and I assure you it is VERY unpleasant.
  8. "John Woodgate" <> schreef in bericht

    Playing Russian Roulette can be worse <G>.
  9. I read in that Winfield Hill <[email protected]_rowland-
    Well, theoretically zero. What actually happens depends on the circuit
    configuration and the device characteristics.
  10. John, which ITU recommendation is that?


  11. I read in that Allan Herriman <allan.herriman.hat
    > wrote (in <9g2qr0dl5q244i2c7vtn3l7jggjek4ou
    >) about 'Differential Output', on Mon, 13 Dec 2004:
    The 126 dB limit for clicks was applied in UK at the time of de-
    regulation, and I am not sure where it came from. The ITU document is
    P360, and I don't have a copy of it. The predecessor had 135 dB for very
    short clicks and 125 dB for longer disturbances.
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