Help understanding RC Power Filter

Discussion in 'General Electronics Chat' started by danielb33, Nov 23, 2012.

  1. danielb33

    Thread Starter Member

    Aug 20, 2012
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    My boss told me to add a RC power filter to my LM324 op amp power input to reduce noise input/output. The op amp power input has very low impedance, so it accepts noise and will pass high noise through to output. Adding a capacitor in order to get rid of output noise makes sense to me, why in the world place a resister before the power in pin?

    He wants me to add a resistor first (50 Ohm) and then place a capacitor after, then comes the input. Resistor is in series with LM317 and LM324, cap is parallel with these and connected to ground.

    I do not understand this at all so I assume I am missing some concepts.
     
  2. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    The RC filter is similar to the LC low pass filter. Rather than an inductor, the resistor is the impedance, preventing surge currents. The capacitor works with the resistor, if you use a filter calculator, you can find the cutoff point of suppressed frequencies.

    Using only a capacitor in parallel with the power source is a way to lower the effective source impedance, typically to keep the voltage steady in a high power system. The resistance in that case is the internal resistance of the battery or power supply. The resistor also helps the capacitor supply current to the circuit, rather than back to the power supply, when the voltage sags.
     
  3. danielb33

    Thread Starter Member

    Aug 20, 2012
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    Thanks much for the reply. Makes much more sense now.
     
  4. Audioguru

    New Member

    Dec 20, 2007
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    An LM324 is very old and is very noisy. Many newer audio opamps have much less noise.
    An LM317 regulator also produces noise but most modern opamps ignor the noise on the power supply.
     
  5. danielb33

    Thread Starter Member

    Aug 20, 2012
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    ...any part numbers to go along with the claim for better op amps?
     
  6. Audioguru

    New Member

    Dec 20, 2007
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    TL071 single, TL072 dual and TL074 quad audio opamps have noise about 5 times less than a lousy old LM324 and the bandwidth is to 100kHz (the LM324 has trouble above only 2kHz).
    The LM324 has horrible crossover distortion but the TL07x opamps have only 0.003% distortion.

    OPA134 single, OPA2134 dual and OPA4124 quad audio opamps have noise about 10 times less than the lousy ones, a bandwidth to 150kHz and distortion of only 0.0008%.

    There are many more good opamps.
     
  7. danielb33

    Thread Starter Member

    Aug 20, 2012
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    Thanks for the reply. I will look into those and most likely revise my schematic!
     
  8. danielb33

    Thread Starter Member

    Aug 20, 2012
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    Also, do you think the LM317 is a lousy voltage regulator???
     
  9. Audioguru

    New Member

    Dec 20, 2007
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    The LM317 voltage regulator is as good as most voltage regulators.
    Most opamp circuits do not need a voltage regulator because the opamp ignors fluctuations of the supply voltage.
     
  10. #12

    Expert

    Nov 30, 2010
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    AG is talking about PSRR...Power Supply Rejection Ratio or SVR...Supply Voltage Rejection ratio. Here's a datasheet of a TL071 that shows 70 db to 80 DB reduction of the power supply noise (depending on exactly which version you use).

    That isn't a perfect, "ignore" but it helps a lot. Just about any regulator chip used to keep the amp safe from over voltage events, combined with the 70db or better Power Supply Rejection, makes most applications like this very easy to keep the background noise down.
     
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