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Solid State Voice Recorder Using Flash MSP430

Hi

I've read application note how to create solid state voice recorder using flash msp430 http://www.ti.com/lit/an/slaa123/slaa123.pdf It says that simple RC filter at the output does the bandwidth limiting with a cutoff frequency approximately 2.7 KHz. On the contrary my calculation show that cutoff frequency is 1.578 KHz. Is there any mistake in schematic design?

  • Vladimir Redzhepov said:
    Is there any mistake in schematic design?

    Did you take in account that resistors of Butterworth filter differ?

  • Vladimir Redzhepov said:
    Is there any mistake in schematic design?

    Could be.

  • Ilmars said:
    Did you take in account that resistors of Butterworth filter differ?

    It is not a Butterworth filter, it is a simple series-R shunt-C low pass filter. The -3dB rolloff point is simply 1/RC.

  • Brian Boorman said:
    It is not a Butterworth filter, it is a simple series-R shunt-C low pass filter. The -3dB rolloff point is simply 1/RC.

    Correct me if I am wrong, but I know following filter as Butterworth filter of Sallen-Key topology:

  • Ilmars

    The answer was about microphone preamplifier and filter but you're talking about serial DAC and filter.

  • Vladimir Redzhepov said:
    The answer was about microphone preamplifier and filter but you're talking about serial DAC and filter.

    You did not mention "microphone" nor "preamplifier" in your initial post. How would I know that you mean input filter by saying following words (pay special attention to word "output"):

    Vladimir Redzhepov said:
    It says that simple RC filter at the output does the bandwidth limiting with a cutoff frequency approximately 2.7 KHz

    Yes, then it seems like mistake. Good for you that you checked it :) Also it would be good idea to test frequency response of whole device during prototyping.

  • Ilmars and Brian thank you for answers and comments!

  • Ilmars said:
    You did not mention "microphone" nor "preamplifier" in your initial post.

    Llmars - You are correct, and it confused me at first as well, until I asked Acrobat to search the PDF for the word "2.7 KHz" and the only hit was in the preamp/filter narrative.

    There it is just an amplifier with an RC tacked on the output.

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