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OPA333: Amplifier for a VU meter

Diego Palomeque
Prodigy 80 points
Other Parts Discussed in Thread: OPA333

Hi, I'm working on a VUmeter around an ARM uC. I would like to read audio signal with an ADC input of the uC.

As the ouput of mp3 portable player or cell phones is too low compared with the 3.3V ADC input I want to add a simple amplifier.

I'm working on this schematic:

My questions are:

  1. What is the amplitud signal (typical) for a portable device?
  2. What is the max gain that could obtain from that kind of amplifier taking into account that I want 0-20KHz?
  3. Could I use a variable resistor instead of R11 to adjust the gain?
  4. Is there another better circuit for that purpose?

Thank in advance!

  • 0
    TI__Mastermind 32395 points
    Hello Diego,

    1. I do not know the typical amplitude from portable music players. This depends on what's the application, what's the device, etc. Whatever this range is will change how much you can gain the signal. Fortuanately, this amplitude range won't impose a requirement for U1's input common-mode voltage range since you have it in an inverting configuration.

    2. OPA333's unity GBW is 350 kHz. If a fc of 34 kHz is desired then the max closed-loop gain achievable is 350/34 = 10.29 V/V. If you change your gain setting resistors to R12/R11 = 17.5 then the closed-loop BW of the amplfier is 20 kHz, in which case you would not need C12 anymore.

    3. You could use a variable resistor for R11 to vary the filter gain.

    4. This is harder to answer without knowing the full application. You will want to consider the ADC you will be driving with U1. It's very possible OPA333 does not have enough BW for this purpose. You could use a higher speed buffer if necessary. TI has collateral covering and learning material for this here:

    www.ti.com/.../data-converters-learning-center.page

    Best,

    Peter Iliya
    Precision Amplifiers Tucson