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www.thinkSRS.com About Lock-In Amplifiers Application Note #3 Lock-in amplifiers are used to detect and measure very small AC signalsall the way down to a few nanovolts. Accurate Reference measurements may be made even when the small signal is obscured by noise sources many thousands of times larger. Lock-in amplifiers use a technique known as phase-sensitive sig detection to single out the component of the signal at a specific reference frequency and phase. Noise signals, at Signal frequencies other than the reference frequency, are rejected and do not affect the measurement. Why Use a Lock-In? Let's consider an example. Suppose the signal is a 10 nV sine wave at 10 kHz. Clearly some amplification is required to bring the signal above the noise. A good low-noise amplifier ref will have about 5 nV/Hz of input noise. If the amplifier Lock-in bandwidth is 100 kHz and the gain is 1000, we can expect our output to be 10