• # EEVblog #528 – Opamp Input Noise Voltage Tutorial

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 HERE

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• Tom

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?

• Jay

Volts RMS or VRMS is the root-mean-square average voltage, which is often used for AC waveforms.

https://en.wikipedia.org/wiki/Root_mean_square

• Tom

Hi Jay,

In Dave’s video at minute 6:45 he explains the calculation of the RMS-Noise 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 e-mail 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 Op-Amp 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)!

• KyranF

It’s because he’s an extremely experienced engineer, and he’s worked on some series projects. He even worked for Altium back in the day.

• Jay

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/root-Hz 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/root-Hz 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 low-frequency audio range. Some opamps have attractive nv/root-Hz 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!

• Donà Enrico

Thanks Dave, another great video.

For a more in-depth treatement I would like to suggest this tutorial series on Op.Amps by Texas Instruments: http://www.ti.com/lsds/ti/amplifiers-linear/precision-amplifier-precision-labs.page?DCMP=tipl&HQS=hpa-pa-opamp-tipl-pr-lp-en

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