Solid State Voice Recorder Using Flash MSP430

Vladimir Redzhepov

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?

over 11 years ago

0 Ilmars over 11 years ago

Guru 46710 points

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

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

0 Brian Boorman over 11 years ago

Guru 25210 points

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

Could be.

0 Brian Boorman over 11 years ago in reply to Ilmars

Guru 25210 points

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.

0 Ilmars over 11 years ago in reply to Brian Boorman

Guru 46710 points

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:

0 Vladimir Redzhepov over 11 years ago in reply to Ilmars

Prodigy 60 points

Ilmars

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

0 Ilmars over 11 years ago in reply to Vladimir Redzhepov

Guru 46710 points

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.

0 Vladimir Redzhepov over 11 years ago in reply to Ilmars

Prodigy 60 points

Ilmars and Brian thank you for answers and comments!

0 Brian Boorman over 11 years ago in reply to Ilmars

Guru 25210 points

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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Solid State Voice Recorder Using Flash MSP430