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You need to use oscilloscope. You can buy 500 MHZ digital oscilloscope on ebay for 700 bucks or you can use PC sound card for low voltage and low frequency.
When you deal with noise you have to understand over what bandwidth the noise is spread. If the frequency spectrum of the noise is flat over a given frequency spread it can be expressed as a fixed volts/per root hertz. You will see this spec for many op amps. (nV/root hertz).
Total equivalent AC rms noise voltage will be V noise (in V per root hz) * square root of bandwidth.
Total noise power in a given termination resistance would be (V per root hz * square root of bandwidth) ^2 / R termination.
Noise figure is 10 * log (1 +( total noise power / KTB)) where K = 1.38E-23 , T = operation temp, B = bandwidth.
I am in the similar situation recently. I want to measure the 100~100kHz output noise of LDO. I use the spectral analyzer to analyze the spectral of output, but I find that the performance of spectral analyzer is very bad in such low frequency. I change the feedback factor but the output noise didn't change at all, which is against the theory. any one has experience of testing output noise of LDO?
thanks in advance!
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