AIC34 noise level record path

Hi Thomas,

In order to improve the SNR value, it is suggested to use a 20kHz low-pass filter and an A-weighted filter. In fact, the SNR value was measured for the datasheet with these filters, as you can see in the next note:

I hope this helps you. If you still have questions, please let me know.

Best regards,

Luis Fernando Rodríguez S.

HI Luis Fernando,

It is not completely clear to me how you want me to implement the low pass filter.

I understand from the datasheet that the digital processing path is not available when both DAC’s are in use and powered.

Do you want me to re-program the ADC high-pass coefficients (N0, N1, D1) and let the filter behave like a low-pass filter? I tried this today but it didn’t help much to reduce noise in the record path.

Anyway, I have ordered the AIC34 EVM board to see if I can get better results with it.

Thanks

Thomas

Hi, Thomas,

You may try with the digital audio processing for record path function. It consists in use the digital filters for the record path, disabling the DACs. You can configure the digital filters to be used as 20kHz low-pass filters. You can use the Coefficient Calculator tool to obtain the coefficients in a easier way. 

Please take a look of DIGITAL AUDIO PROCESSING FOR RECORD PATH section in datasheet for details.

I hope this helps you. If you still have questions, please let me know.

Best regards,

Luis Fernando Rodríguez S.

Hi Luis Fernando,

I bought the AIC34 evaluation board and did some simple noise tests.
The unconnected differential input "Line 2" has a noise level of around -58dB at 0dB input attenuation and +59.5db PGA Gain in differential mode. Connecting a 200pF capacitor as short between the differential inputs increases the noise level to around -42dB.
This is a noise level increase of around 16dB.
I tried different passive networks on the differential input but they all behave the same.
Do you have an explanation for that?

Can you point me to some application notes discussing low noise input circuitries?


Thanks
best regards
Thomas

Hi, Thomas,

Could you describe how you are making the noise measurement? Probably, a filter could be required to make these tests. Please take a look of the following document: Out-of-Band Noise Measurement Issues for Audio Codecs. It is about the effects of filtering for measurement purposes.

I hope this helps you. If you still have questions, please let me know.

Best regards,

Luis Fernando Rodríguez S.

Hi Luis Fernando,

my setup is very simple.
I am not using the line and headphone outputs at all, also the DAC is not powered.

The evaluation board is connected to a Windows PC and I am doing a recording with 59.5dB PGA gain and 0dB input attenuation.
The line 2 input is configured in differential mode and the ADC High-Pass filter is enabled to remove some DC offset.
There are some tools available to analyze the incoming audio sample or the recording.

The noise level is around -58dB when the differential input (in my case line 2) is left unconnected and around -42dB with a 220pF capacitor across the differential input pins. I also tried a solid wire as short between the diff. inputs but the noise level increase is the same.

I am wondering what is wrong with my setup or if this is the best noise performance I can get with maximum gain.

best regards
Thomas

Hi, Thomas,

To use the differential inputs, it is necessary to route them only with an individual capacitor on each line (as the EVM schematic shows). The differential lines must be connected without any additional component to the EVM.

Best regards,
Luis Fernando Rodríguez S.