We have TDA7719 audio processor and OLED display connected to the same
I2C bus as I2C slaves. Both the devices are driven by MCU which is I2C
master. After TDA7719 is initialized with I2C for input selection and
volume, it starts working. The MCU keeps refreshing the display by
continuously writing data to the display through the same I2C bus. This
I2C noise is getting coupled into the audio signals and I can hear the
rhythmic noise from the speakers proportionate to the display
refreshing. If the display is paused the noise also pauses.
What could be the issue? Appreciate any help.
Both MCU and TDA7719 are powered by separate regulators. Input to the
TDA is from MCU which has built in DAC. Problem in the audio source from
MCU+DAC is already ruled out as direct connection to the amplifier
bypassing TDA is outputting clean audio output.
Thanks
markj
wow, a FUN project!
Do you know the magnitude of the sound? Are you hearing 60dB down, or
80dB down? Could be 'GND' noise coupling in. The 'exra' demand of the
power could be shifting the gnds out from under your audio stuff.
If you provide an email address I can send a .zip file to, I'll send you a
sample of the results of a PCB Layout Tool I created to solve those EXACT
problems. [gmail accounts seem to reject a .zip file and never tell
anyone] The tool was created during the design of seismic monitoring
electronics and the gnd noise was kept to less tha 1/4 LSB. Also did a lot
of work to make ALL the power filters lossy over their WHOLE bandwidth,
even into the GHz ranges. We're talking about a filter that looks like a
huge series resistance and a short to gnd. A lot of people don't notice
that most of the filters 'recommended' by the mfgr actually go into high
impedance at certain spectral locations. That translates to having
absolutely NO bypass at those tones. For example, bypass down to 0.1 ohm
great!, but most of those filters at some frequency suddenly go to 10 and
even 100 ohms. No problem *if* you have no frequencies there, but a big
problem if you do! Put a scope probe on your power and you'll see 'ringy'
waveforms, that's the effect.
Another place to look is magnetic fields coupling across traces. MagFields
are INSIDIOUS! Your scope will barely show them, yet they can induce
audible stuff everywhere.
then there's the AM modulated HF that gets rectified somewhere, too.
I'm probably preaching to the choir on all this. Right now, I'd go forthe
GND shifting.