Analog circuits & signal conditioning
Op amps & negative feedback
Let the output help correct the error.
An op amp amplifies the difference between two inputs with very high open-loop gain. Negative feedback returns part of the output to the inverting input and can make closed-loop behavior depend mainly on external components. The inputs become nearly equal only while the loop operates properly within its limits. A comparator makes a decision about which input is larger; it is not simply an op amp used carelessly without feedback.
Gain, filters & frequency response
A circuit can treat slow and fast changes very differently.
Frequency response describes gain and phase versus frequency. Low-pass, high-pass and band-pass filters select different parts of a signal. Bode plots use logarithmic frequency and often decibels for gain. For a simple dominant-pole voltage-feedback op amp, gain-bandwidth is approximately constant. Large signals can also hit slew-rate limits even when a small-signal bandwidth calculation looks acceptable.
Noise, grounding & signal quality
Not every unwanted wiggle has the same cause.
Thermal noise comes from random charge motion. Other errors include interference, offset, flicker noise and quantization. Signal-to-noise ratio compares useful signal power with noise power. Ground is a reference and return network, not a magic zero-voltage sink. Shared impedance, large loops and poor decoupling can couple unwanted signals between circuits. Differential measurement rejects only the common-mode part within its limits.