Define input-referred noise in a signal chain, and identify the common sources in an op-amp circuit.

• A. The equivalent input voltage/current noise that would produce observed output noise
• B. The noise due to power rails
• C. The actual output noise
• D. The sum of all noise sources in the system

Answer: (A)

Input-referred noise is the equivalent noise at the input that would have to be present to produce the observed noise at the output, given the circuit’s gain. This lets you compare noise performance on a common input basis, regardless of how much gain the stage provides. To find it, you take the measured output noise and divide by the gain (for a voltage-gain path), translating all noise sources (voltage noise, current noise through impedances, and resistor thermal noise) into a single input-referred value.

In an op-amp circuit, the usual contributors are the op-amp’s own input voltage noise and input current noise (the currents flowing through source and feedback resistances create additional noise voltages), plus the thermal (Johnson) noise of resistors in the input and feedback networks. Flicker noise at low frequencies can also be important. Power-supply noise can affect the output as well, but it’s not part of the standard input-referred noise definition unless you explicitly map it through the circuit to the input.

So the correct idea is that input-referred noise represents the equivalent input signal level that would explain the output noise, with the other options describing either the output noise itself, a general sum, or rail noise that isn’t mapped to the input path. For example, with a gain of 20 and 200 µV of output noise, the input-referred noise is 10 µV.