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AC coupling on High speed Lines

Abhi P Choksi , 08-13-2019, 06:06 AM
(1)I want to know that how AC coupling capacitor benefits High-speed Lines such as PCIe, SATA, etc?

(2)Particularly which high-speed interfaces require AC coupling? Which technical parameter decide that this Type of High-Speed interface will require AC coupling?

(3)It's ok to follow the recommended value of AC coupling capacitor from the design guide but, any formula to calculate this?

Paul van Avesaath , 08-13-2019, 07:40 AM
1) it depends on the internal structure of the device you are conecting to. most datasheets supply how it is done so you can have a look
2) it is always stated in the datasheet of the component.
3) yes, and as a (very) basic rule of thumb the higher the speeds the smaller the caps. but this also depends on what type of signal you are using.
Abhi P Choksi , 08-13-2019, 10:43 PM
Thanks for calarification.
robertferanec , 08-16-2019, 08:09 AM
- Simply to say AC coupling capacitors are used to remove DC element from the signals - you do not want to have DC in specific high speed signals (that makes these signals worse).

Why? I believe, for example - it may be more inaccurate to read the differential pairs if they have DC (especially if DC is different between negative and positive signal then you will be getting different places where signals are crossing the important points for reading) - by removing DC you move the signals on the same level. Also when high speed differential signals have DC, maybe it could influence them (I am not sure about this, but have a look at this video when Yuriy is talking how in the places where differential pairs have same polarity how the phase and speed of the signal may be influenced: https://youtu.be/ZyoqhJB_E9Q?t=1328 ). This is what I think ... but maybe there are other reasons for this.

- These capacitors are usually in differential pair signals

- For value, I would follow datasheets and reference schematics, however very often 100nF caps are used.
Abhi P Choksi , 08-19-2019, 10:05 PM
Hi @robertferanec
Thanks for your answer.

Can you list out High Speed Interfaces which require AC coupling Capacitor.

Paul van Avesaath , 08-20-2019, 12:01 AM
there is not such a list, because it is component specific. READ THE DATASHEET..

@robertferanec there are some DC coupled higspeed signals I have come accros so i think the internal structure of the IC is current based for those.... but as a practice I always put in a 0402 footprint.. this way i can choose if it will be a capacitor or a resisitor and you can experiment.
robertferanec , 08-20-2019, 01:13 AM
there are some DC coupled signals
- yes there are, I corrected my answer
anovickis , 09-03-2019, 12:19 PM
Generally use AC coupling as drive levels will vary, but p-p voltages will be correct

If your design spreads to more than one board - especially if one can be off while the other is on - _always_ AC couple in that case

I'll post details on that in a separate thread for some recent debugging fun

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