Thanks Mike for your reply.
First of all i apologize for goofing up part number. I am infact using LMP7721
**broken link removed** . This has max input bias current of only 20fA and typical of 3fA.
Let me clarify something more about my circuit:
1. I am infact interested in getting 60Hz. It is not noise for me it is the signal i am interested in.
2. My Circuit is battery powered by 3.7V battery.
3. Currently I do not have faraday-cage or shield around it. I am going to try it.
How do you make a resistor which is high enough for your needs, but low enough to supply the required bias current?
>> I don't think we can get resistor value that high. Even if we can get thermal noise (Johnson noise) in resistor will screw things up.
TI has a solution that is to connect JFET in reverse and take advantage of JFETs low gate leakage. Check
http://focus.ti.com/lit/an/sboa058/sboa058.pdf. TI recommends Siliconix’s 2N4117A JFET for high input impedance applications.
My problem is 60fA is too high for me. Also it turns out that even etched PCB itself can not provide enough resistance between supply pins and input pin. Leakage can easily go upto 3fA and destroy amplifier's operation.
So now i am building circuit in air basically op-amp is in mid air and i am soldering respective pins directly to its pins.
But aside from my circuit, i have a fundamental question:
"
Why op-amp need input bias current? Is input bias current is just a leakage from input port of op-amp IC to ground pin inside IC or Is it fundamentally a requirement of circuit inside IC?"
If someone can shed light on it, it will greatly help me increase my knowledge.
Thanks & Regards