TPA2028D1: When AGC is enabled, Noise issue

Hi SS Jung,

Could you send me some output graphs when this condition happens?

Best Regards
José Luis Figueroa
Applications Engineer

Hi José Luis.

I attached testing waveform.

As I mentioned, Only On AGC enabled situation, tic.....tic.....tic (periodic) noise is occurred.

When 1Kz audio input's volume is down and AGC is no activated, noise is disappeared.

When 1Kz audio volume is up and AGC is activeated, noise is occurred.

Please let me know how to remove this noise.

 

Noise on AGC.ppt

Hi SS Jung,

It seems to be decoupling issues as the device output is getting high peaks. This would explain the output noise.
Make sure that the device is working at least with two decoupling capacitors. First one should be of 1uF to help with higher frequency transients, spikes or digital hash on the line and the other one should be of 10uF to help with lower frequency noise signals. It is very important to place them as close as possible to the device.

Best Regards
José Luis Figueroa
Applications Engineer

Hi Jose Luis.

I already tested decoupling as 1uF and 10uF on PVCC Line. This is not solution about the issue. (They have another white noise issue too, so I already tested this method and going on)

Point is AGC function.

This noise is only on AGC activated status.

Please refer below image.

A is output is smaller than gain limiter level. At this time noise is not occurred.

B is output is larger than gain limiter level. At this time noise is occurred.

Volume Control

A->B : No noise -> Noise occur

B->A : Noise occur -> Noise disappear

Noise like Tic...Tic...Tic (periodic), something like horse running sound.

This noise have some connection with attack/hold/release time?

Please check register setting.

Hi SS Jung,

The output signal will be clipped for exceeding the gain limiter level and that could be producing output noise.
The device output signal should always be smaller than the gain limiter level.

Best Regards
José Luis Figueroa
Applications Engineer

Hi Jose Luis.

As you know output signal amplitude is limited according to gain limiter level.
And this operation is TPA2028D1's AGC function.
And This AGC Function is customer's necessary function.

Upper your mention is when output is exceeded gain limiter level and AGC is activated, this noise is normal?

And Customer has some questions for impedance matching.
1. TPA2028D1's Input Impedance??
2. TPA2028D1's Output Impedance?? (I already know Zout(output impedance in shutdown mode) is 2K Ohm)
They want to know operating output impedance with 8 Ohm speaker.
3. Some document about Audio AMP Impedance Matching .

Thank you

Hi SS Jung,

That noise shouldn't be normal under this condition. Could you provide me a schematic and a layout?
1) You can refer the Figure 28 of the amplifier's datasheet to find a graph of Input impedance vs Gain.
2) The TPA2028D1's output impedance is 0.75Ω.
3) Unfortunately, we don't have a document about Impedance Matching.

Best Regards
José Luis Figueroa
Applications Engineer

Hi Jose Luis.

I attached Sch and Layout image.

This noise (Tic....Tic....Tic..., Periodic noise) exist only on AGC operation, case of exceed limiter level.

Please check register setting again.

SS Jung,

I have observed the layout and I cannot find the decoupling capacitors, Are they been placed on another layer?

Best Regards
José Luis Figueroa
Applications Engineer

Jose Luis.

Yes.
C10 = 10uF, C12 = 1uF

SS Jung,

I know that the decoupling capacitors are C10 and C12, but I couldn't find them on the PCB layout because there isn't any label that describes each component. Could you provide me the PCB layout file? or Could you add the labels to the PCB layout picture?

Best Regards
José Luis Figueroa
Audio Applications Engineer

Hi Jose Luis.

I attached PCB Layout file with component label.

Please refer.

And I also attached noise sound file.

This sound file was recorded as the input source of 1KHz was raised or lowered so that AGC was turned on / off.

You can hear the noise like tic..tic...tic during AGC Enabled section.

I thins this noise is register setting problem. Because this noise only occurs when the output exceeds the limiter level and AGC is turned on.

Please re-check register setting (register setting is already uploaded)

My customer want below function.

1. Fixed Gain 16dB

2. Limiter level 9dBi. (To prevent the speaker output from exceeding 3W when the input volume increases)

3. Input Source is normal analog audio signal.

4. Output Speaker is 4Ohm Speaker.

AGC Noise.m4a

SS Jung,

Could you provide me the audio signal level approximate used in the application?

Best Regards
José Luis Figueroa
Audio Applications Engineer

Hi Jose Luis.

Input Audio Signal level is MAX 760mV.

SS jung,

I have performed some tests under these conditions and the device didn't produce any weird sound. I set an input signal in order to get a THD+N=10% so that the AGC works with the signal level and after I toggled the limiter, but the weird sound didn't appear.

Best Regards
José Luis Figueroa
Audio Applications Engineer

Hello. Jose.

Thanks for your reply.

I think Your upper test is not AGC working state. And my test is also same result.

But when fixed gain is set higher than limiter level and AGC is activated, weird sound is occurred.

I checked this issue using TPA2028D1 EVM.

I attached Register Setting, HW setting and weird sound file. Please refer.

You can hear weird noise sound like tic...tic...tic...(this is periodic noise) in sound file.

This tic..tic...tic sound is only at AGC activated.

If fixed gain is set lower this weird sound is disappeared.

But Limiter level is set lower (AGC is off), this weird sound appeared again.

And on this situation, when Release time is set higher, weird sound period is changed.

According to upper result, I think this weird sound tic...tic...tic... is occurred only in AGC is ON.

In sound file, you can hear weird sound while AGC On/Off state changing and release time changing.

AGC Noise on 1KHz Tone.m4a

Is this weird sound on AGC ON normal symptom?

If no, how can I debug? What is debugging point?

Please re-check and help me.

Thanks.

Hi SS Jung,

I used a song the first time that I performed the test, I think AGC was not activated and I didn't notice the problem.
I performed the test under the conditions that you mentioned and I got the same weird noise. As you mentioned, It seems that this noise is produced when the output signal is higher than limiter level.
I noticed that the limiter is being activated all the time. But, The limiter is designed to suppress high spikes in order to prevent speaker damage from occurring and it should not be working all the time.

Best Regards
José Luis Figueroa
Audio Applications Engineer

Hi Jose.


As you mentioned, when output signal is higher than limiter level, AGC is activated and protect speaker.
As I know this function is AGC function and TPA2028D1 support this.
So my customer selected TPA2028D1 and they need to AGC function.

If the user goes up the limiter level by raising the volume, we want AGC function is working and protect speaker.

According upper your reply, Is this weird sound normal operation when it exceeds the limiter level?

Hi. Jose,
My customer need to AGC function.
And their desired setting is must using AGC and highest output signal is exceeded the limiter level.

Many TI Audio AMP has the AGC function.
These all AMP has this weird sound in AGC enable status?
This sound is normal symptom?

We are so many tested to reduce spike on AGC status, but this weird sound is still occurred.
PVDD's decoupling cap and input output filter, etc.

We want to know this symptom is normal operation or not.

Thank you

SS Jung,

As I mentioned the AGC is working all the time in this application, but that should be only working when the output signal exceeds the limiter level due to transients high spikes until the output signal comes back to its normal level (the normal level shouldn't exceed the limiter level) and it should only take a few seconds. The AGC shouldn't be working all the time.
We can't guarantee a right behavior from the device if you don't follow the given guidelines in the amplifier's datasheet.

Best Regards
José Luis Figueroa
Audio Applications Engineer