RC4580-Q1: Why adding a filter capacitor to the output will reduce the SNR?

Ansel Song

Part Number: RC4580-Q1
Other Parts Discussed in Thread: RC4580

Hi team,

Why adding a filter capacitor (1nF) to the output will reduce the SNR? In the actual test, the customer added a 1nF capacitor to the output with an SNR of 80dB, and without the capacitor, the SNR was 86dB. Why does adding capacitance decrease SNR? Thanks.

Best regards,

Ansel Song

over 4 years ago

0 Luis Fernando Rodriguez S. over 4 years ago

TI__Guru* 84915 points

Hi, Ansel,

I think this could be associated to the phase shift modification due to a pole modification. I suggest to check this article below. It mentions that an output capacitor may have risk to cause oscillation when it is not correctly selected.

https://e2e.ti.com/blogs_/archives/b/precisionhub/archive/2016/01/22/amp-up-your-cans-op-amps-for-headphones-cap-load-drive-part-2

Please let me know if you have additional questions or comments.

Best regards,
Luis Fernando Rodríguez S.

0 Ansel Song over 4 years ago in reply to Luis Fernando Rodriguez S.

TI__Expert 3890 points

Hi Luis ,

From the material you provided, it seems to affect the phase margin, but it does not seem to affect the low frequency curve. I only need to calculate the SNR within 20kHz. Could you share some method to get the SNR? And why the output cap effects the SNR? Thnaks.

Best regards,

Ansel Song

0 Luis Fernando Rodriguez S. over 4 years ago in reply to Ansel Song

TI__Guru* 84915 points

Hi, Ansel,

The method to get the SNR in our audio devices is through a level comparison with a full-scale input waveform (or maximum amplitude waveform) and no input. With our audio precision tools, we follow the indications of this website:

https://www.ap.com/technical-library/signal-to-noise-ratio-snr-dynamic-range-and-noise/

Regarding the SNR decrease due to the capacitor, most of the operational amplifiers have limited capacitive load capabilities. In this case, the customer is using the capacitor as a load, which may result in instability. I would suggest to add a small series resistor at the amplifier output and see if the SNR level changes.

Best regards,
Luis Fernando Rodríguez S.

0 Ansel Song over 4 years ago in reply to Luis Fernando Rodriguez S.

TI__Expert 3890 points

Hi Luis，

Thanks for your reply, I think I could do some simulation with RC4580, but I cannot find the pspice module form ti.com, could you kindly share the module?

Best regards,

Ansel

0 Luis Fernando Rodriguez S. over 4 years ago in reply to Ansel Song

TI__Guru* 84915 points

Hi, Ansel,

Unfortunately, all the the simulation models that we have for these devices are available in the RC4580.

http://www.ti.com/product/RC4580/toolssoftware

The Pspice module has not been developed and there are no plans to develop it in the following months.

Best regards,
Luis Fernando Rodríguez S.

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RC4580-Q1: Why adding a filter capacitor to the output will reduce the SNR?