# Calculating Input Noise Voltage

• posted

I was just checking out various opamp datasheets. I encountered this opamp:

In the few lines of text, it says that the input noise voltage is 9 nV/ sqrt(Hz) at 10kHz. But in the table at the bottom of the page, it says that the input noise voltage is 18 nV/sqrt(Hz) at 1kHz.

How did they calculate this?

I'm curious because if I were to use an opamp (not necessarily this one) then I would like to know how much input noise voltage it contributes for a certain bandwidth, for instance audio band from 20Hz to 20kHz. There was another discussion that I saw (don't remember the link) that mentioned that at one particular design, the amplifier was designed with a 40dB gain and the input noise got amplified to about

100mV.
• posted

"MRW"

** Groper fool alert.

** See figure 19 ??

See how the EIN goes up dramatically at frequencies below 3 kHz ?

Go check some other op-amps - discover they are not all like that.

** Normally just multiply the typical or 1 kHz EIN figure by 141 (= sq rt 20,000)

....... Phil

• posted

Thanks, Phil! I'll take a look at other datasheets. Got any opamps you can recommend for audio?

• posted

"MRW"

** See: