Electronics > Beginners
Why is noise input-referred?
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ZeroResistance:
Noise spec are sometimes given in datasheets as some uV or mV figure but they sometimes quote it as input-referred noise.
eg.
Input-Referred Noise: 4 μVPP (150 Hz BW, G = 6)

Why is noise referred to input? What is the significance or the thinking behind it?

My understanding is that  if I want to measure noise of a circuit I would short the input or inputs to ground and then measure the output for an signals?
Is this the method of measuring "input-referred noise"?
Siwastaja:
It removes (or tries to remove) the gain from the equation.

If you increase gain, you amplify the noise as well. So now you have more noise, and a different figure. So you'd need a curve for output noise vs. gain. It happens, this curve would be nearly a straight line. So you can just express input referred noise as a single number, and multiply it with your actual gain.

It's still expressed for a given gain (like G = 6 in your example), because they need to test it with some test circuit, and want to specify what they used. Furthermore, the output noise isn't perfectly accurately gain * input referred noise - so the curve we talked about earlier isn't exactly straight, and maybe doesn't start from 0. Which is why they still need to give a different figure for different gains, but at least they are closer to each other, and often they just get away with specifying one value.
ZeroResistance:

--- Quote from: Siwastaja on June 21, 2019, 03:08:58 pm ---It removes the gain from the equation.

If you increase gain, you amplify the noise as well. So now you have more noise, and a different figure. So you'd need a curve for output noise vs. gain. It happens, this curve would be nearly a straight line. So you can just express input referred noise as a single number, and multiply it with your actual gain.

--- End quote ---

In the datasheet eg. I mentioned as
Input-Referred Noise: 4 μVPP (150 Hz BW, G = 6)
so it seems that they have measured this noise at a gain of 6. Right or wrong?
Siwastaja:
Yes, I edited my post to address this question while you were replying.
ZeroResistance:

--- Quote from: Siwastaja on June 21, 2019, 03:12:37 pm ---Yes, I edited my post to address this question while you were replying.

--- End quote ---

Would it be correct to say that input-referred noise is the noise measured on the output of a circuit with its inputs shorted and the gain set to 1?
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