What is "coupling" and "decoupling"

[flyankh]

coupling and decoupling in cap

Hi all

I am reading some book about ASIC.It always said "coupling" and "decoupling"
What are they mean?I can't understand it.Somebody can help me?

Regards

flyankh

 

[ppenday]

coupling is the parasitic capacitors that exists between metal layer

Decoupling
There are instances where the power distribution between stages cannot be sufficiency bypassed. In this case, the designer might be tempted to use several different power supplies. However, by supplying the DC power to each stage through a separate inductor or "choke," while also bypassing to ground that stage, the effect is the same. That is to say, the choke offers a high impedance path to any errant signals or noise between stages, while offering a very low resistance path to the DC power: this is known as decoupling. Active devices such as voltage regulators can also be used for decoupling stages

 

[flyankh]

Thank you very much ,ppenday
I have understood the "coupling",but what is the "stage" means?What is it standing for?

Regards

flyankh

 

[mady79]

capcitor's which couple the AC/alternating currents and block dc called coupling caps .Coupling cap is like a High pass filter action .It is placed between two nodes which want to couple only AC components .

Decoupling/bypass cap is like a low pass filter which smooth out the high frequency ripples .It is placed from output to ground .

 

[flyankh]

Thank you ,mady79
Now I know the "coupling" and "decoupling" now.In fact it is a way to refuse or only allow the high frequency singal/noise come though,right?

Regards

flyankh

 

[mady79]

You are right flyankh .
coupling caps would be useful in circuits which operate on different common mode voltages .So to just couple AC components coupling caps are used .Decoupling/bypass caps are used in loop filter's in PLL ,Regulator circuits .

 

[ravig]

A good example of coupling caps would be Bipolar amps. You might have seen bipolar amps with two or three stages. There will be coupling caps in between the stages that only allow the AC signal to pass through. The bias levels are blocked and each stage is individually biased.
Even in an IC, we call the capacitance between two adjacent metals as coupling capacitance. The capacitance is formed because of the signal(AC) level variations in the metals.
Obviously decoupling caps are used wherever you want to bypass the noise or some other high frequency components.
In any case the cap passes only AC. The names coupling and decoupling depends on their usage.

 

[chinito]

Even at board level, one uses coupling and decoupling caps. Coupling caps are used, for example, to extract the differential portion of the signal (i.e. LVDS signal, you could care less about the bias point). Similarly, decoupling caps are used on the board for power and gnd lines to get rid of high freq noise on the them