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JohnsonMiller , 02-14-2019, 01:47 AM

Hi guys,

In electrical isolation of part of circuit using isolator IC (Like ADUM4224), I have seen a capacitor placed between two ground, please see the image:
People insist to place that capacitor, I am wondering what is the reason? and what that X1/Y2 mean? Is it especial (form/material point of view) capacitor?

Paul van Avesaath , 02-14-2019, 05:47 AM

image is not working.. please load as attachment..

JohnsonMiller , 02-14-2019, 07:18 AM

Here it is,

Paul van Avesaath , 02-14-2019, 07:42 AM

When using an isolated GND it has to connect to main GND somewere.. to have a good reference.. but to keep noise out it can done with a capacitor.. normally it is a very small caps

i don know about the X1/Y2 maybe some one else has an idea about that.. i know about X2Y capacitors https://www.johansondielectrics.com/...ing-capacitors
which i use in PSU development, but not like the example you have..

in high speed transmission you see this also.. they are called stitching caps. but then it is used to brige gaps in planes ..

robertferanec , 02-15-2019, 12:00 PM

On board, there may be more reasons to connect different grounds with capacitors - for example if there are some digital signals, capacitor can create return path. Or it can for example help with ESD protection. You may need to see the PCB layout or rest of the schematic to know how the capacitor is used.

JohnsonMiller , 02-17-2019, 03:15 AM

My question is when we do isolate grounds, dose not make sense to connect grounds again with capacitors. Sure in DC those GNDs are separated, but in AC and high frequency those GNDs are connected. Any comment?