
EEVblog #528 – Opamp Input Noise Voltage Tutorial
Posted on September 28th, 2013 5 comments
Dave explains one of the most confusing parameters in an opamp datasheet, Input Noise Voltage Density, that mysterious nV/RootHz figure.
Along with different types of opamp noise, corner noise frequency, and how to calculate output noise spectral density.
And how to use a Dynamic Signal Anayser to measure the Power Spectral Density response over frequency of various opamps.
Forum HERE5 responses to “EEVblog #528 – Opamp Input Noise Voltage Tutorial”

First of all, thank you for this great video!
A question regarding what you already pointed out in the video: You selected V/sqrt(Hz) on your DSA but wanted Vrms/sqrt(Hz). If I understood correctly the Noise Voltage Density was in Vrms/sqrt(Hz) what is V/sqrt(Hz) and how is this related to each other?
Volts RMS or VRMS is the rootmeansquare average voltage, which is often used for AC waveforms.
https://en.wikipedia.org/wiki/Root_mean_square
I hope that answers your question.

Hi Jay,
I’m sorry your link does not answer my question or I just don’t see it.
In Dave’s video at minute 6:45 he explains the calculation of the RMSNoise value (vRMS) for a given bandwidth from the voltage noise spectral density which he points out is in vRMS/sqrt(Hz).
Lateron in the video Dave uses the dynamic signal analyzer (DSA) to get the voltage noise spectral density plot. At minute 26:19 he changes the vertical axis unit from dB*vRMS/sqrt(Hz) to v/sqrt(Hz). The video overlay says he ment to select vRMS/sqrt(Hz).
So there again is the question: When the noise spectral density is measured in vRMS/sqrt(Hz) and given in every plot in all datasheets what is v/sqrt(Hz) and why doesn’t it matter for the video’s further relative measurements and when does it matter? What is this v(olt) in the v/sqrt(Hz) at all? Is it a peak value, is it a mean value is it the statistically random noise value which was measured just at the moment here?



Dave – Apparently I can’t send you an email so here it is … I have followed you for several years now. I am VERY impressed with your knowledge of theory, applications,PCB design,and production issues. I am really surprised that you even know what an OpAmp is, much less all the detailed specs. Granted my college education in EE was back in the early 70′s, but I am now having more fun with electronics (since I retired) than I ever have. You have certainly contributed to that fun. Keep it up dude (Bloke)!

I learned about noise specs for opamps many years ago by reading datasheets and application notes. It was a few years B.I. (Before Internet), and it was very difficult then to find even one clear explanation of opamp datasheet specs!
I was confused about the nV/rootHz spec for a long time and just used it for comparing differnt opamps. The breakthrough for me was when I learned how to compute total noise by using the frequency bandwidth of my circuit, as Dave shows. It seems the reason they publish the spec in nV/rootHz is to allow the manufacturer to provide one number that designers can use to calculate total noise in any circuit.
Also, I’d like to add that there can be a big difference in 1/f noise in various opamps, so as Dave says, don’t just look at the banner specs — look at both the noise spec *and* the graph showing 1/f noise if your circuit works near DC or in the lowfrequency audio range. Some opamps have attractive nv/rootHz numbers, but the 1/f noise can be a nasty surprise due to a high corner frequency, high 1/f noise levels, or both.
This video was a good refresher, and I even picked up a few new things. Good job!
Leave a reply

Tom September 29th, 2013 at 07:32