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Thread: Maximum signal level on audio inputs.

  1. #1
    Junior Member
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    Maximum signal level on audio inputs.

    Hello,
    I'd like to connect some analog circuitry to the audio board's (Rev D with Teensy 4.0) line level inputs. And I have a couple of questions. First of all. What is the voltage range for the accepted line in signals (From Teensy GND)? So, they get converted to values from 0 to 65,535 (entire 16bit audio library signal range). (I want to cover all of the range for maximum quality). If that isn't exactly an option, what is the recommended voltage range? And most importantly. Does the audio board have overvoltage protection on the line level inputs or do I need to implement it myself?
    Thanks for the help.

  2. #2
    Senior Member PaulS's Avatar
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    What is the voltage range for the accepted line in signals
    According to the SGTL5000 datasheet, the typical LineIn voltage is 1.0VRMS:

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    (on page 6, it states maximum 2.83VPP)

    As far as I can tell there is no overvoltage protection on the LineIn pins.

    Regards,
    Paul

  3. #3
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    The voltages are converted to signed values (the range -32768..32768) with the Audio library. This is normal for
    audio ADCs and DACs (but not for non-audio ADCs and DACs).

    The SGTL can output upto 24 bits, the Audio library only takes the 16 most significant bits.

    The SGTL5000 may well be fried if you over-voltage it, though the series capacitors provide some protection.
    Its unclear if there's any on-chip protection, probably some anti-static protection diodes, but I doubt they'd
    survive anything like a 10Vrms incoming signal from an audio mixer for instance(!)

  4. #4
    Senior Member ETMoody3's Avatar
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    You can set this with a function in the SGTL5000 control code.

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  5. #5
    Senior Member PaulS's Avatar
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    Perhaps nitpicking, but setting 0: 3.12Vpp does formally violate the SGTL5000 spec.

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    Paul Stoffregen apparently measured these voltages to be OK [see control_sgtl5000.cpp] and the chip will survive this voltage but distortion figures might not be guaranteed.

    @MarkT: any thoughts about this?

    Paul

  6. #6
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    Its a 3.3V chip, the inputs can go upto the rails in CMOS. The max input is likely the sigma-delta clipping
    point (sigma delta converters often clip somewhat before the rails).

    The datasheet not being self-consistent is not exactly rare! Hoever it might be something to do with gain
    variability - unlike other converter types the gain of a sigma-delta converter can be subject to process
    variation and temperature dependence. Some of those figures may be maximums, some typical.

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