I am designing simple monitoring application circuit, used 3analog inputs. Im confused that how to choose decoupling capacitor for that dc voltage stability and analog inputs. What kind of capacitor need to add, how to choose?
Without any particulars, a 1 µF ceramic capacitor placed physically close between the power and ground pins of the device you are trying to decouple is a good generic answer.
Back in the pleistoscene, 100 nF capacitors were commonly used. 100 nF would work for most decoupling situations too, but the choice of capacitance was dictated more by what was possible to make cheaply. Today with multi-layer surface mount ceramic caps being cheap and plentiful, there is little harm in increasing the capacitance a bit. With proper layout, these will have less series inductance than the 100 nF thru hole caps of ancient legend.
Take a look at the impedance curves of a family of modern capacitors. You will see there is little penalty in using higher capacitance. 1 µF is enough to to the job, and are jellybean commondity items at normal logic voltages.
In one case I used a specific model of 100 pF cap for decoupling because it was a RF application, and that cap had a lower impedance at the RF frequency than any other I could find.
Capacitors used for decoupling means the supply noise source is suppressed by the lower impedance at high frequency so that does not couple to the amplifier signal. Generally low series resistance, low inductance, bulk multilayer ceramic and film types are best in SMT. You don't care about tolerance as much impedance vs f, which for audio and low power is easy to accomplish because the current is low. Cost is a major factor, when are used in many places, so decoupling types are cheap, but low impedance over range of interest. This used to be occupied by two cap types in older voltage regulators with more ac noise. One electrolytic large for low frequency noise and one small for RF noise. Now it depends on your supply noise what you need, as there is no one solution. Too little and too much uF, can be bad for some regulators and don't care for others.
It is wise to follow recommended practice in datasheets.