I2C noise into the audio signals

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

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.

2C bus as I2C slaves. Both the devices are driven by MCU which is I2C maste r. After TDA7719 is initialized with I2C for input selection and volume, it starts working. The MCU keeps refreshing the display by continuously writi ng 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 s peakers proportionate to the display refreshing. If the display is paused t he noise also pauses.

A 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 TD A is outputting clean audio output.

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

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.

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.

OK, similar situation. You might try lowpass filtering the inputs of the TPA, maybe just some small caps to ground to test the idea. The front end of the TPA could be rectifying fast spikes from the I2C.

You could also try slowing down the I2C edges.

--

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

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.

Typical design mistake.

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.

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

Vladimir Vassilevsky DSP and Mixed Signal Designs

Short circuit on i2c lines? MCU driving it active hi?

Right. Short the input and see if the noise is still there. Then use an external supply for the TDA. Then...

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?

Nah, let's call it slightly suboptimal :-)

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

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

--
Regards, Joerg 

http://www.analogconsultants.com/

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.

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.

speed could be slowed down (for the audio IC).. and then add some RC filte rs on the I2C input wires. Then see if that reduced the interference. (or is that a silly idea?)

lator 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 effe cts because of 'bad' ground returns. (and maybe other means as well.) The inductive and capacitive stuff will depend on the edges. Whereas the commo n ground can be like a resistive addition... So if you slow down or speed u p the I2C edges and it gets better (or worse)* that tells you something. I f 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 s hot might help.

George H.

*in trouble shooting I often find it easier to make it worse.. before I can make it better. :^)

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.

--
Regards, Joerg 

http://www.analogconsultants.com/

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.

On a sunny day (Tue, 10 Dec 2013 18:15:07 +0200) it happened snipped-for-privacy@downunder.com wrote in :

I have had this problem i2c breakthrough into audio once. Golden rule: Keep your audio impedances low. Keep i2c tracks away from (not paralel to) audio tracks. Yes differential audio helps but is not always possible.