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PCM1681: Problem about the active filter.

Part Number: PCM1681
Other Parts Discussed in Thread: OPA2353, OPA337, OPA2134, OPA1622

I have several questions about PCM1681.
Please refer to the datasheet, figure 26 and figure 33 show the recommended active filter circuits, my question is:
(1): can both filter circuits be used as active filter circuit? Which one is the priority?
(2): For the circuit with OPA2353, if all of the VOUT1 to VOUT7 are used, can I connect all of the non-inverting input of operational amplifier to the output of OPA337? Or is this a must?
(3): For the circuit with OPA2134, if I only connect the positive power supply, can the active filter circuit work?
Thanks.

  • Hi, and welcome to the e2e forums!

    1. I am not sure what you mean by priority. The difference in the circuits is only that figure 33 requires a bipolar, dual supply (±5V, for example) and figure 26 only requires +5V.  The gain on the two active filters is slightly different as figure 26 has less supply headroom so the full-scale output must be limited to prevent output clipping. Both are active filters.

    2. You can use one buffer of the VCOM voltage for all of the outputs.  Basically, the VCOM pin is just a voltage divider, so it cannot source any current without distorting the output of the DAC.  For that reason, it must be buffered.

    3. The input of figure 33 is ground-centered and the circuit has a negative gain.  If you only supplied it with a positive rail, the output would essentially be clipping into ground.  The op-amp would not be operating in its linear region, so the output would be heavily distorted.

    Thanks,
    Paul

  • Hi Paul,

    Thanks for your detailed explanation.

    I still have one question:

    When is the circuit in figure 26 suitable, and when is the circuit in figure 33 suitable? Are there any obvious differences other than the gain?

    thanks.

  • The circuit in figure 26 is useful when you only have a single supply.  The VCOM offset voltage allows you to keep the output centered at +2.5V, so no negative supply is needed.

    If you have a bipolar supply, ±5V for example, then figure 33 can be used as no VCOM offset voltage is needed.  In this case, the output is centered around 0V.

  • Thanks for your reply, I'm clear about it.

    What's more, what kind of sound can be connected to the output of active filters? A sound or a speaker? Can you give me a suggestion about this?

  • Hi,

    The output of this amp is capable of being used as a "line-out" function, where this circuit would then be connected to a high-power amplifier to drive speakers or headphones.

    In addition, you could select an op amp that is capable of driving more power to drive headphones or some small speakers.  Consider the OPA1622 or 1656.

  • Hi Paul,

    I have understood this problem.

    Thanks.