TPA3111D1: Limit Bandwidth

CJ Carnicle

Part Number: TPA3111D1

The customer is using the TPA3111 and wants to adjust the device to limit the bandwidth of the output.

They are using these AMPs for a limited bandwidth of up to 1KHz. They are using the app note ferrite (80 ohm) and cap (1000 pF) values which are for the full audio bandwidth.

Can they limit the bandwidth of this amp to closer to the 1KHz max that they are putting into it?

They are getting some high frequency audible noise through the system.

over 8 years ago

0 Jeff Kohnle over 8 years ago

TI__Expert 6985 points

Hello, Shawn can help you with this question! Thanks, Jeff

0 Shawn Zheng over 8 years ago

TI__Mastermind 24835 points

Hi CJ,

Ferrtite bead + capacitor filter is used for EMI supression, but not be for bandwith limit usage. Please use inductor + capacitor filter on the audio output. Actually we have a very detailed LC low pass design document and tool. Please refer to http://www.ti.com/lit/an/slaa701a/slaa701a.pdf and http://www.ti.com/lit/zip/slac729. I believe with them you could design appropriate output filter for your application.

Actually 1kHz cutoff frequency is very low, so you need to use very large inductors and capacitors for the output filter. Why don't you try to do the LPF in DSP or Codec?

Best regards,

Shawn Zheng

0 Jeff Kohnle over 8 years ago in reply to Shawn Zheng

TI__Expert 6985 points

Hello CJ! Did this work for you and can we close this thread? Thanks, Jeff

0 CJ Carnicle over 8 years ago

TI__Prodigy 320 points

Thanks for the info, they are looking into the document.

The issue is that they are using an LPF digitally but are then using a sigma delta ADC with a 2 stage RC reconstruction filter so the high frequency noise is still fairly high into the AMP input. They are looking for other opportunities to filter it out. Will the document help with this?

0 Shawn Zheng over 8 years ago in reply to CJ Carnicle

TI__Mastermind 24835 points

Hi CJ,
Thanks for the update and explanation. I think there is a sigma-delta DAC in front of the audio amplifier in your application, right? It use noise-shaping techniques to improve in-band SNR performance at the expense of generating increased out-of-band noise. Some sigma-delta DAC has on-chip LFP integrated, while some needs external low-pass filters. Usually we use a RC filter on the output of the DAC to suppress the high-frequency noise(refer to www.ti.com/.../pcm5122.pdf). Here is a document about sigma-delta DAC performance evaluation www.ti.com/.../sbaa055.pdf You can also use a active MFB LPF on the output of the DAC. Please refer to the Page33 on this datasheet. www.ti.com/.../pcm1789-q1.pdf About active filter design, please find these documents: slea048 sloa049b sloa093

Best regards,
Shawn Zheng

0 Jeff Kohnle over 8 years ago in reply to Shawn Zheng

TI__Expert 6985 points

Hi CJ! DId this help you? Thanks, Jeff

0 CJ Carnicle over 8 years ago in reply to Jeff Kohnle

TI__Prodigy 320 points

Hey Jeff, yes this helped. Appreciate the support from the team!

We can close this now.

Regards,

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TPA3111D1: Limit Bandwidth