TAS3251EVM: The purpose of the RC network on DAC_OUT pins

Hi Sia,

RC network is used for low pass filter which limits the output bandwidth, in order to eliminate high frequency or RF noise. Resistor is also used for limiting the output current, and in combination with the capacitor, it will determine the cut-off frequency of the low pass filter by the formula: fc=1/(2*3.1416*R*C). Thanks

Thanks, some follow up questions:

1. This looks like a differential low-pass filter. Ambient RF noise would be common mode, not differential. Is high frequency noise coming from the DAC_OUT pins?
2. Should these be placed near the DAC_OUT pins or the SPK_IN pins?
3. What's the point of the 100k resistors? 

Thanks,

Hi Sia,

Below are my answers to your queries. Thanks.

1. Yes, the high-frequency noise is most likely originating from the DAC_OUT pins. The DAC conversion process can generate high-frequency noise (such as switching noise or harmonics), which exists in a differential mode rather than a common-mode mode. While ambient RF noise is typically common-mode, this differential filter is specifically targeted at the differential noise generated by the DAC itself. Therefore, the RC low-pass filter is used here to attenuate the high-frequency differential noise produced at the DAC_OUT pins, ensuring a clean output signal.

2. The filter should be placed as close as possible to the DAC_OUT pins. This allows the noise to be filtered at its source before it can propagate into the subsequent circuit (like the amplifier or speaker input SPK_IN), reducing its impact on the overall system. Placing filters close to the noise source (the DAC) is a fundamental best practice in PCB layout for effective noise suppression.

3. The 100k resistors serve several potential purposes:

  ◦ Setting the cut-off frequency: They work in combination with the capacitors to determine the cut-off frequency of the low-pass filter (fc = 1/(2πRC)).

  ◦ Impedance matching: They can be used for impedance matching between the source and the load in high-impedance circuits, reducing signal reflection and distortion.

  ◦ Current limiting: They act as a current limiter for the output, protecting downstream components (like a speaker or amplifier) from excessive current.

Hi Sia,

Hope above explanation helps to answer your questions, are there any further feedbacks? Thanks.

Hi Peter, thanks your answers make sense. 

Hi Sia,

Thanks for your confirmation. Have a Good Day!