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Thread: Ferrite 600 and noisy MUXs on T4.1

  1. #1

    Ferrite 600 and noisy MUXs on T4.1

    Hello I'm moving from a T3.6 to a T4.1. My MUXs have a lot of noise when reading analog signals now (from the 600MHz MCU?) I have a separate ground plane for the pots I'm reading. I want to re-introduce the ferrite 600 on my board that is missing from the T4.1.

    I assume 600 is 600ohms at 100MHz? Are there any other tips for removing noise, my PCB has 100nF caps on the pots, which have helped previously. Thanks.

  2. #2
    Senior Member
    Join Date
    May 2015
    Consider adding an external ADC with differential (or pseudo differential) inputs and a proper low pass filter. Even better if you can switch the input signal to a known value, allowing a pulsed signal, which helps with low frequency 1/f and 60hz noise. Think of this as "calibrate a few micro seconds before reading" or more technically, a lock-in amplifier.

  3. #3
    Senior Member PaulStoffregen's Avatar
    Join Date
    Nov 2012
    The first and easiest thing to check is settling time, between when you change the mux pins and when you actually read the voltage. If you have a call to delayMicroseconds(), it should be ok. But if the timing is only by how fast code executes, you'll have much less settling time with Teensy 4.0. Use delayMicroseconds() so you get consistent settling time.

  4. #4
    Thanks for the replies. I looked at the 4067 datasheet and wondered if it was nearing the limit of how fast it could be reliably read. I should probably stop being lazy and put a scope on it and get a better idea of what delay to introduce.

  5. #5
    Senior Member oddson's Avatar
    Join Date
    Feb 2013
    Isle in the Salish Sea
    ANY delay is usually enough, I've started at five microseconds but one seems to do it.

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