What does that mean?

I have read this http://digital.ni.com/public.nsf/allkb/0F81C33977056A7D862566F1005CAA71
Does it mean that whenever we have to ac couple pin to GND we have to connect a capacitor between pin & gnd just like decoupling capacitor.

 

No, here is what our friends at NI say:

Hardware: Multifunction DAQ (MIO)

Problem:
What are AC and DC coupling?

Solution:
AC coupling consists of using a capacitor to filter out the DC signal component from a signal with both AC and DC components. The capacitor must be in series with the signal. AC coupling is useful because the DC component of a signal acts as a voltage offset, and removing it from the signal can increase the resolution of signal measurements. AC coupling is also known as capacitive coupling.

DC coupling allows both AC and DC signals to pass through a connection. When using DC coupling, no additional capacitor is added to filter the signal.

To AC couple a signal with an NI data acquisition (DAQ) device, insert a capacitor in series with the signal, between the positive lead of the signal and the positive analog input channel on the DAQ device. Wire the negative lead of the signal directly to the corresponding negative analog input channel. This removes the DC component from the signal, which centers the AC signal at 0 volts.

If we have an AC signal riding on a DC component level using a cap in the line will block the DC component. Here is the NI pdf on the subject.

Ron

 

@Reloadron yes the link I got what ac & dc coupling mean.
But in my circuit I have to ac couple a pin to GND. Does that mean that I have to connect a capacitor between pin & gnd like a decoupling capacitor.

 

That's what it sounds like, but "in my circuit" makes this look like you sent us to a reference that has nothing to do with your circuit. How are we supposed to guess what YOUR circuit needs?

 

@Reloadron yes the link I got what ac & dc coupling mean.
But in my circuit I have to ac couple a pin to GND. Does that mean that I have to connect a capacitor between pin & gnd like a decoupling capacitor.

Anytime since a capacitor passes AC and blocks DC you connect AC to to a capacitor that is going to a ground you have effectively shorted your AC signal to ground and thus this makes no sense. DC power pins use a decoupling capacitor to ground but coupling an AC signal to ground makes no sense. Would really need to see a schematic or drawing of what you are talking about.

Ron

 

@Reloadron yes the link I got what ac & dc coupling mean.
But in my circuit I have to ac couple a pin to GND. Does that mean that I have to connect a capacitor between pin & gnd like a decoupling capacitor.

The term "ac couple" in this context is ambiguous. The term "couple" is used when we want to pass on signals for further use - amplifying, filtering, etc. "bypass" or "decouple" is the term used when "ground" comes in.

You need to show a schematic or a explanation for us to infer what actually you mean in this particular case.

If we do away with the terminology, "Yes" is the answer to your question.