Q.
How is voltage noise of a LDO measured?
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A.
LDO noise is measured two ways 1) Total RMS noise over a specified bandwidth, and 2) Noise spectral density at several frequencies over a specified range. To measure the RMS noise, the noise of the LDO is amplified with a low noise preamp before being measured with a true RMS voltmeter. The preamp bandwidth is 10Hz to100kHz, and is set by fourth order filters. The preamp gain is 60dB with a noise floor of less than 1uVrms referred to the input. The noise spectral density measurements are made with a lock-in amplifier with a 1Hz bandwidth. This is essentially a narrowband receiver that can be tuned over a wide range of frequencies. The noise spectral density per root Hertz is measured with a voltmeter and a plot of the Noise Spectral Density vs Frequency is generated.
