Decoupling Capacitor

Lookup the subject: FILTERING, AC-to-DC.

Read these:

How to select decoupling capacitors - All About Circuits Forum

AN1325 "Choosing and Using Bypass Capacitors" - Intersil

Decoupling capacitor - Wikipedia, the free encyclopedia

Read through the links provided by Lyn, and do the search per 70AARCUDA, and read.

To simplify, yes the OPAMP may have power supply noise rejection, but it's not zero. Then think of what the amplifier does as it drives the output, with noise and signal from it's input. Since it is not driving infinite impedance, the current driving the OPAMP's load, is also being conducted back to the supply rails. And anything on the supply rails, is seeing this noise, and causes other circuits to work with this noise. Decoupling is to stop the supply rails from being swamped by all the circuit noises. But it also provides additional power supply noise filtering, as well the power supply sees the noise of it's loads, and depending on how well it's regulation and control feedback damping, minimizes the noise effects from changing the output i.e. amplifying supply rail noise.

To keep electrical noise from the power supply from getting into your analog signal.

Decoupling capicitor is used to decouple one part of electrical circuit. Decoupling or byepass capicitor is used to dampen the AC and to filter undesired noise from power supplies. Another term used for the bypass capacitor is a filter cap.

Ask yourself the question:-

What're the characteristics of a capacitor:-

1. They store energy

2. They block DC

You can build on these two if you've asked yourself the question and, taken time to think about it.

Now apply these characteristics to the circuitry you're trying to evaluate.