Original question:
Dear all,
I am doing a project on CMOS op amp, I went through many data sheets, what i assumed was Input bias current is the current flowing through the input volltages and input impedance is change in input voltage divided with change in input current. Is my assumption correct, How to measure input Bias current and Input impedance. Help me please.
Thanks in advance
Since the input impedance implies a resistive and reactive part, you must seperate the two:
1. An input common-mode resistance, Rin_cm, may be calculated from the slope of graph of IB vs Vcm - see below:
Rin_cm = ΔVcm/ΔIB = 5V/0.75pA = 6.7e12 or 6.7Teraohm
2. The common-mode input capacitance, Cin_cm, can be determined based of the corner frequency roll-off caused by added large input resistor, R1, and sweeping the frequency - see below:
Since f=1/(2*∏*R*C), then C = 1/(2*∏*R*f) = 1/(6.28*1e6*25,000) = ~6.4pF
