A quiet, well regulated supply is important for optimum performance with a number of circuit applications. Voltage controlled oscillators (VCOs) and precision voltage controlled crystal oscillators (VCXOs) respond to small changes in their supply very quickly. Phase-locked loops (PLLs) require a stable supply as signal on the supply translates directly to phase noise in the output. RF amplifiers require quiet supplies as they have little to no ability to reject supply variations and regulator variation will appear as unwanted side bands and lower the signal-to-noise ratio. Low noise amplifiers and analog-to-digital converters (ADCs) do not have infinite supply rejection and the cleaner the regulator output is, the higher their performance.

In addition to noise, one must also consider the supply rejection capabilities of the linear regulator. Poor rejection from a linear regulator will bring switching regulator residue or other unwanted signals through, corrupting the hard work done to ensure a clean supply. Extremely low noise from the regulator is worthless if poor supply rejection brings enough signal through to swamp noise levels.

Linear regulators such as the LT3042 are now in production with much lower output voltage noise levels. While the family of regulators released that operate with approximately 20μVRMS noise in the 10Hz to 100kHz band, the LT3042 is now available with noise levels of 0.8μVRMS across the same frequency band. With output noise levels of 0.8μVRMS, this noise floor is now unacceptable; the regulator itself operates at noise levels only slightly above the measurement circuit. This translates to almost 20% error, making the measurement circuit too significant a factor to be able to measure signals with confidence.

The exacting performance offered by linear regulators like the LT3042 provides exceptionally quiet supply rails for sensitive systems. Verification of DC performance from such a device is usually not a tricky proposition. Critical parameters such as noise and supply rejection are not as easy to measure at such high performance levels. Careful attention must be paid to the smallest of details in measurement circuits, connections, board layout and equipment. What were once small errors that could be ignored (compared to the signal being measured) are now first-order error terms. The high PSRR performance delivered shows that signals are not being transmitted through the device itself, but through magnetic coupling. Every detail must be scrutinised to ensure fidelity of measurements and provide solid results that are trustworthy.

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