Why is a resistor added after the DC blocking capacitor of the op-amp input?
The opamp input pins have a small bias current. Without R2 in your example the capacitor would gradually charge up or down eventually bringing the pin to V+ or V- and the circuit would no longer work as intended.


  I have read somewhere that R2 added provides the dc return to ground however this would reduces the overall input impedance of op-apm.


Yes, so it is important to understand the effect on the circuit.


  I/p impedance must be high for op-amp.How to solve this problem?


Not quite right. The op-amp's input impedance is high by design. It will have no objection to being connected to quite a low resistance. For example, if the op-amp is used to measure the voltage drop across a current shunt the resistance might be &lt;&lt; 1 Ω.







simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. R1 allows OA1 to measure the current through R1, a 0.1 Ω current shunt.