TL074 Equivalent Noise Specification

Hi Mark, this is my very first post so I'm really hoping that it isn't a lemon! Noise specs are a real barrel of laughs at times. The 1kHz figure is just for that one frequency and it varies a lot over the spectrum. Also, it is actually a figure of power density and, as such, it can also be quoted in units of volts squared per hertz (no root sign.) I personally think this is where a lot of the confusion comes in. To make it of more practical use, and this is where the second figure of 4uV comes from, it is often quoted for a particular range of bandwidth. Now, 10Hz to 10kHz is near enough 10kHz for practical purposes, and if we take its square root we get 100. We then multiply this by the noise figure given in per root Hertz (18nV in this case) and we get 1800nV, or 1.8uV over the given bandwidth. Note that this is not the same as the figure provided (4uV) and this is because the value at 1kHz is, by definition, not a constant as I mentioned earlier. In particular, for general purpose op amps, it rises sharply at low frequencies (see Fig 16 in the data sheet) so making the wide bandwidth noise much more than the figure simply based upon that at 1kHz. I apologise in advance if this has made matters worse! Cheers, Mike.