Don’t let the title scare you off: “equivalent input noise” is just another way of measuring how noisy an amplifier is. The nice feature in this case is that it takes the gain out of the equation, so it doesn’t matter if the gain of the amplifier is 20dB, 26dB, 32dB, or 38dB – you can still compare apples to apples.
Perusing the datasheet of a class-D amplifier from one of the most highly regarded class-D manufacturers in the world (no, not me…yet), I came across the following noise specification:
5nV/sqrt(Hz) equivalent input noise
Well, is this quiet, noisy or what? As it turns out this is extremely quiet, as quiet as a 1.5kΩ resistor just sitting there on its own in fact! What is even more amazing is that this amplifier features an input impedance of 100kΩ! In other words right at the input of the amplifier is something that should be generating about 41nV/sqrt(Hz) of noise, yet the amplifier only features 5nV/sqrt(Hz) of noise!?
How is this possible you may ask? Well, it simply isn’t. To the best of my knowledge people have not yet determined how to cancel out random noise. Reading the datasheet a bit more it seems that the amplifier features a “minimal path voltage mode” input with an impedance of 660O. That must be it. It must be this “minimal path voltage mode” input that is used for purposes of the noise measurement, but is this the input typically used by customers?
This does strike me as a bit of obfuscation. The other parameters of the amplifier are very good, so why give your customers this useless noise data? How about simply providing the dynamic range or signal to noise ratio for the 100kΩ input – the one the customer will most likely use, or at the very least provide understandable noise data for all of the inputs?
For those interested, here’s how you calculate the noise of a resistor:
$$v_{n}=(4kTR)^{1/2}$$
k = Boltzmann’s constant
T = temperature in Kelvins
R = resistance of the resistor