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Thread: need advises for designing low noise circuits

  1. #1
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    need advises for designing low noise circuits

    Hello,

    I have been playing with Teensy for a while now, and I need some help to build less noisy circuits.

    I mainly build musical instruments and MIDI controllers with a lot of pots, switches, sensors, multiplexers... I noticed that the more complex my circuits are, the more noise I get especially when reading pots and sensors. I need some guidelines to build "quite" circuits. Here are a few questions:

    - what are the best practices to filter and stabilise the power supply ? I noticed this is often the main source of noise.
    - best practices to reduce pots noise ? Why adding more pots lead to more overall noise ?
    - how to design PCBs ? Do I have to make ground planes ?
    - how to avoid ground loops ?
    - I am not sure to understand the benefit of using analog GND. How to use tis feature with pots for instance ?

    This is a lot of questions... I don't ask for ready to use solutions, but if one can give me links to documents and threads I could study, that would be nice.

    Emmanuel

  2. #2

  3. #3
    Quote Originally Posted by tschrama View Post
    I was just about to suggest ESP as well - lots of examples with good explanations, which helps in determining what are the critical design considerations (power supply, type of capacitors to use, etc.)
    Sadly this kind of knowledge seems to be going away with most of the audio processing now done in the digital domain. Nevertheless, the end results will invariably depend on the quality of the analog front end

    Marc

  4. #4
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    Quote Originally Posted by emmanuel63 View Post
    Hello,

    I have been playing with Teensy for a while now, and I need some help to build less noisy circuits.

    I mainly build musical instruments and MIDI controllers with a lot of pots, switches, sensors, multiplexers... I noticed that the more complex my circuits are, the more noise I get especially when reading pots and sensors. I need some guidelines to build "quite" circuits. Here are a few questions:

    - what are the best practices to filter and stabilise the power supply ? I noticed this is often the main source of noise.
    Linear regulators, not switch-mode. Decent decoupling for audio (ceramic 100nF + 100uF electrolytic for instance).
    - best practices to reduce pots noise ? Why adding more pots lead to more overall noise ?
    For sudio signal pots use low values, 10k to 1k sort of range, to keep Johnson noise down. DC-blocking capacitor is
    normally mandatory on the wiper to prevent DC bias currents being interrupted as the wiper moves.
    (this can make massive rail-to-rail noise spikes).
    - how to design PCBs ? Do I have to make ground planes ?
    Star grounding is good, ground plane is much easier and pretty good in practice. Keep outputs away from inputs
    so output currents don't pollute input ground/rail traces. Power joins the board at the output end, not the input end.

    Fitting the whole unit in a metal box is strongly recommended. Diecast boxes are great for this as they are easy
    to work with (drill, saw, file). (Yes they look pretty dull unless painted)
    - how to avoid ground loops ?
    Run the grounding linearly along the board alongside the signal path, ground connected to power supply at
    output end of the board as mentioned. Ground the enclosure to the signal ground at one point only. This means
    for instance that either the input jacks or output jacks should have isolated shield connections.
    - I am not sure to understand the benefit of using analog GND. How to use tis feature with pots for instance ?
    The whole of the analog circuitry should be separately powered and grounded from the digital. Digital circuits
    generate broadband noise across the whole spectrum from audio to microwaves. With any kind of processor this
    noise will be modulated in many different ways as the program runs, which is usually very audible if it leaks
    into the analog side of things.
    Where analog and digital have to meet (ADC, DAC), should be the only place the grounds join. You can reduce
    the coupling between grounds here by using a small resistance like 10 ohms in series with an RFC/ferrite bead.
    This is a lot of questions... I don't ask for ready to use solutions, but if one can give me links to documents and threads I could study, that would be nice.

    Emmanuel

  5. #5
    Senior Member
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    The easiest way to reduce noise is to go with the trend - do everything possible in the digital domain.

  6. #6
    Senior Member
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    Dec 2018
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    Thank you very much MarkT.
    I am going to redo my PCB design following your rules.
    Emmanuel

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