When calculating the output noise (rms) of the OPA4340 when used as a voltage buffer, I wanted to include voltage noise due to current noise (through the input resistors).
But the current spectral noise in the datasheet increases as frequency increases, which I am not used to.
How do you find the Noise Voltage due to Op-amp Current Noise in this case?
What equation is used?
Darren,
A low frequency input current noise spectral density may be calculated using a following formula: Inoise=(2*q*IB)^0.5 so for IB=1pA, Inoise=(2*1.6e-19*1e-12)^.5 = 0.6fA/rt-Hz. However, with the increase of frequency, common-mode input capacitance (6pf) causes an increase in IB resulting in the Inoise increase until UGBW of OPA340 roll it off - see below.
However, since the thermal noise of 1Mohm and 1Gohm resistor is 126nV/rt-Hz and 4uV/rt-Hz, respectively, the effect of the input current noise in CMOS amplifiers like OPA340 is negligible - see below.
Simulating the output noise shows the total integrated noise of 51uVrms for R1=0 and 69uVrms for R=1Mohm and 1Gohm - see below.
Below I have attached the Tina-TI schematic for your convenience.
