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I2C noise into the audio signals

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+DACis already ruled out as direct connection to the amplifier bypassing TDA is outputting clean audio output.

Thanks
markj
 
R

RobertMacy

Jan 1, 1970
0
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.
 
Do you have any opamps after the TDA?





--



John Larkin Highland Technology Inc

www.highlandtechnology.com jlarkin at highlandtechnology dot com



Precision electronic instrumentation

Picosecond-resolution Digital Delay and Pulse generators

Custom timing and laser controllers

Photonics and fiberoptic TTL data links

VME analog, thermocouple, LVDT, synchro, tachometer

Multichannel arbitrary waveform generators

No John. Output of TDA is fed to TPA3110D2 amplifier.
 
G

George Herold

Jan 1, 1970
0
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.
Isn't the obvious path via the I2C wires? I was wondering if the I2C speed could be slowed down (for the audio IC).. and then add some RC filters on the I2C input wires. Then see if that reduced the interference. (or is that a silly idea?)
I guess the first job is to figure out how it's getting in.

George H.
 
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.


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.

You can send the zip to markjsunil at live dot com

The power supply should be OK as LM317L SOT89 pack can supply 100ma and TDA7719 max current is 35ma.
 
R

RobertMacy

Jan 1, 1970
0
You can send the zip to markjsunil at live dot com

The power supply should be OK as LM317L SOT89 pack can supply 100ma and
TDA7719 max current is 35ma.

sent, look forward to your comments

not to be too ascerbic, but what does that PS chip have to do with
anything? From memory that chip's not so good either.

the ratings you quoted are DC, check the spec sheet and you'll see a
fairly high impedance even as low as 10KHz. That's why caps are usually
added. BUT! a lot of these 3 terminal regulators go through a huge output
impedance spike for your whole PS system depending on the value of cap you
use. TI makes a great series of 3T's that let you include the cap in the
feedback loop minimizing that effect.

John mentioned slowing the I2C edges. That can solve a lot of problems
even with the source coming from different reasons. ie, capacitive feed,
gnd spikes, ps spikes, magnetic feed, etc etc.

Be careful shorting energy to ground WITHOUT providing a way to starve the
energy supply, else the problem can simply exacerbate itself. From years
of working in EMC mitigation learned the hard way that bypassing doesn't
always do what you think it does. *IF* you don't include some series
resistance. Just have to change your thinking from voltage in time, to
ENERGY in time, then you can find where to stomp on unwanted signals.
After all, at those higher frequencies, nature doesn't care if it's
voltage OR current flying around.
 
J

Johann Klammer

Jan 1, 1970
0
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
Short circuit on i2c lines? MCU driving it active hi?
 
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.
Isn't the obvious path via the I2C wires? I was wondering if the I2C speed could be slowed down (for the audio IC).. and then add some RC filters on the I2C input wires. Then see if that reduced the interference. (or is that a silly idea?)
I guess the first job is to figure out how it's getting in.

Right. Short the input and see if the noise is still there. Then use
an external supply for the TDA. Then...
 
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.
Isn't the obvious path via the I2C wires? I was wondering if the I2C speed could be slowed down (for the audio IC).. and then add some RC filters on the I2C input wires. Then see if that reduced the interference. (or is that a silly idea?)
I guess the first job is to figure out how it's getting in.



Right. Short the input and see if the noise is still there. Then use

an external supply for the TDA. Then...


George H.

shorted the inputs and noise is still there. It already has separate regulator but the power source is same for both MCU regulator and TDA regulator. I will try with 2 separate power sources.
 
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.




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.
Isn't the obvious path via the I2C wires? I was wondering if the I2C speed could be slowed down (for the audio IC).. and then add some RC filters on the I2C input wires. Then see if that reduced the interference. (or is that a silly idea?)
I guess the first job is to figure out how it's getting in.



Right. Short the input and see if the noise is still there. Then use

an external supply for the TDA. Then...


George H.

shorted the inputs and noise is still there. It already has separate regulator but the power source is same for both MCU regulator and TDA regulator. I will try with 2 separate power sources.

Can you disconnect I2C from the device?
 
J

Joerg

Jan 1, 1970
0
Vladimir said:
Typical design mistake.

Nah, let's call it slightly suboptimal :)

Don't run active communication at, to, or near analog processing ICs.
Run control I2C from separate I2C port or at least isolate I2C by
3-state buffer or multiplexer.

Seconded. That is most likely the only way to make the noise go away in
this case.

Slowing I2C edges, splitting power/ground may help to some extent;
however the right solution is changing design to isolate I2C.

In the audio spectrum it will not help because you can't round them this
much.
 
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.

My first reaction is to look for common paths (PCB tracks or wires)
for both audio ground and digital ground.

Some digital noise in the 300-6000 Hz frequency range could be easily
heard 60-80 dB down from the analog audio signal. Thus, any digital
signal that causes a microvolt voltage drop in the common ground paths
can be audible.
 
In the audio spectrum it will not help because you can't round them this
much.

One way to get rid of any digital intrusions into analog signals is to
make sure that the digital signal does not contain loooong sequences
of "1" or "0", which could create frequency components below 20 kHz.
Manchester coding etc. could help a lot.
 
G

George Herold

Jan 1, 1970
0
shorted the inputs and noise is still there. It already has separate regulator but the power source is same for both MCU regulator and TDA regulator.. I will try with 2 separate power sources.

OK then some other path. So I'd try and figure out what the path might be.There's common ground stuff, capacitive coupling and then inductivce effects because of 'bad' ground returns. (and maybe other means as well.) The inductive and capacitive stuff will depend on the edges. Whereas the common ground can be like a resistive addition... So if you slow down or speed up the I2C edges and it gets better (or worse)* that tells you something. If it's roughly independent of the edges then perhaps look for common groundreturn issues. Can you see the pickup on your 'scope? Posting a 'scope shot might help.

George H.
*in trouble shooting I often find it easier to make it worse.. before I canmake it better. :^)
 
J

Joerg

Jan 1, 1970
0
One way to get rid of any digital intrusions into analog signals is to
make sure that the digital signal does not contain loooong sequences
of "1" or "0", which could create frequency components below 20 kHz.
Manchester coding etc. could help a lot.

With I2C you don't have much of a choice, the timing windows aren't like
with SPI. Also, the human ear is super sensitive to noises that aren't
smooth. Even a tiny pulse at low duty cycle but repeated within the
audio spectrum can be heard.
 
OK then some other path. So I'd try and figure out what the path might be. There's common ground stuff, capacitive coupling and then inductivce effects because of 'bad' ground returns. (and maybe other means as well.) The inductive and capacitive stuff will depend on the edges. Whereas the common ground can be like a resistive addition... So if you slow down or speed up the I2C edges and it gets better (or worse)* that tells you something. If it's roughly independent of the edges then perhaps look for common ground return issues. Can you see the pickup on your 'scope? Posting a 'scope shot might help.

In professional high quality audio applications either galvanic
isolation (audio transformer) or at least differential signaling is
used to avoid all of the ground resistance and most of the
inductive/capacitice problems on connections between equipments in
separate boxes.

Maybe the OP should look at the situation from this point of view even
for communication between PCBs.
 
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