why we required to ground any part of circuit such as in AND gate we ground the input terminal?
advanced thanks.

 

Is it all the input terminals or only one input terminal. We are required to place a ground in a circuit so that the circuit will have a reference.

 

Gates in general and op amps/comparators require their inputs grounded or given a "1" if you are not doing anything with them. This is very fundimental to circuits and design.

If an input is left open the device can oscillate, causes lots of other problems.

 

Most gates that you will use these days are CMOS or similar. This means that they have very high impedance inputs. A high impedance input can easily be influenced by nearby voltages or fields. Even your hands moving around can charge either input of a gate. These small charges move the voltage around on the input and can cause the IC to switch rapidly between '0' and '1' if conditions are right. All this switching causes excess power dissipation (wasting it since you aren't using the gate) This switching can also influence other gate inputs that aren't connected to '1' or '0', causing them to switch, and so-on. All this switching creates noise, and with enough noise, real, connected inputs can be disturbed, causing the circuit to malfunction, for no apparent reason. Also, if enough voltage builds on the open input (static electricity easily exceeds 1000V), that input is destroyed. Most likely, this leads to destruction of the whole chip, without any obvious reason.

Tying any unused inputs to '1' or '0' provides a low impedance connection to the gate, and it cannot be influenced by anything, preventing all these problems. Unused outputs must be left unconnected, because they are low impedance from inside the chip and will not float away like the inputs.